Stiffening elements and methods of using stiffening elements in current return networks

ABSTRACT

A stiffening element that includes one or more integral current flowpaths may include: a first layer including carbon-fiber-reinforced thermoplastic plies; a second layer including one or more glass-fiber-reinforced thermoplastic plies; and/or a third layer including aluminum. The third layer may form an outer surface of the stiffening element. The third layer may form at least part of the one or more integral current flowpaths. A method of using a stiffening element that comprises one or more integral current flowpaths as part of a current return network for a stiffened structure may include: selecting the stiffening element that includes a first layer including carbon-fiber-reinforced thermoplastic plies, a second layer including one or more glass-fiber-reinforced thermoplastic plies, and/or a third layer including aluminum; and/or routing current from the current return network through the one or more integral current flowpaths of the selected stiffening element.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/378,931, filed on Dec. 14, 2016, in the U.S. Patent andTrademark Office (“USPTO”); and is a continuation-in-part of U.S. patentapplication Ser. No. 15/378,982, filed on Dec. 14, 2016, in the USPTO;the entire contents of both of which are incorporated herein byreference.

FIELD

The subject matter described herein generally relates to stiffeningelements and methods of using stiffening elements. More particularly,the subject matter disclosed herein relates to stiffening elements thatcomprise one or more integral current flowpaths. The subject matterdisclosed herein also relates to methods of using stiffening elementsthat comprise one or more integral current flowpaths as part of currentreturn networks for stiffened structures.

BACKGROUND

Stiffening elements may comprise fiber-reinforced thermoplastic prepregplies. The fiber-reinforced thermoplastic prepreg plies may be, forexample, glass-fiber-reinforced thermoplastic prepreg plies. Thefiber-reinforced thermoplastic prepreg plies may be, for example,carbon-fiber-reinforced thermoplastic prepreg plies.

Stiffening elements also may comprise, for example, aluminum, glasscomposite, and/or carbon composite layers. The glass composite layer(s)may comprise the glass-fiber-reinforced thermoplastic prepreg plies. Thecarbon composite layer(s) may comprise the carbon-fiber-reinforcedthermoplastic prepreg plies. Such stiffening elements may fall, forexample, in the fiber metal laminate (“FML”) category. FMLs may exhibitspecific advantages when compared to simple metal structures. Suchadvantages may include, for example, improved resistance to corrosion,fatigue, fire, and/or impact. In addition or in the alternative, suchadvantages may include, for example, integral electromagnetic effects(“EME”) conductivity. In addition or in the alternative, such advantagesmay include, for example, specialized strength properties and/or reducedweight per given volume.

Many industries, such as the aerospace, automotive, defense,electronics, maritime, and rail-transport industries, continually seekto push the boundaries of what has come before in structures such asaircraft, bridges, buildings, cars, locomotives, missiles, rockets,ships, stiffening elements, submarines, submersibles, towers, traincars, and trucks. In particular, as cost and weight reduction may beprimary factors, relatively inexpensive, lighter composite materials maybe substituted for relatively more expensive, heavier metals in thosestructures. But the composite materials may not be able to be used inthe same manner as the metals, particularly with regard to electricalfunctions. Thus, there is a need for improved structures, such asstiffening elements.

SUMMARY

The present disclosure is directed to stiffening elements and methods ofusing stiffening elements.

In some examples, a stiffening element that comprises one or moreintegral current flowpaths may comprise: a first layer comprising aplurality of carbon-fiber-reinforced thermoplastic plies; a secondlayer, adjacent to the first layer, comprising one or moreglass-fiber-reinforced thermoplastic plies; and/or a third layer,adjacent to the second layer and opposite to the first layer, comprisingaluminum. The third layer may be configured to form an outer surface ofthe stiffening element. The third layer may be configured to form atleast part of the one or more integral current flowpaths.

In some examples, the third layer may comprise 1000 series aluminumalloy.

In some examples, the third layer may comprise 1100 aluminum alloy.

In some examples, the plurality of carbon-fiber-reinforced thermoplasticplies may comprise thermoplastic resin.

In some examples, the thermoplastic resin may comprisepolyetheretherketone (“PEEK”) or polyetherketoneketone (“PEKK”).

In some examples, the one or more glass-fiber-reinforced thermoplasticplies may comprise thermoplastic resin.

In some examples, the thermoplastic resin may comprise PEEK or PEKK.

In some examples, the plurality of carbon-fiber-reinforced thermoplasticplies may comprise first thermoplastic resin, the one or moreglass-fiber-reinforced thermoplastic plies may comprise secondthermoplastic resin, and/or the first thermoplastic resin may be thesame as the second thermoplastic resin.

In some examples, the plurality of carbon-fiber-reinforced thermoplasticplies may comprise first thermoplastic resin, the one or moreglass-fiber-reinforced thermoplastic plies may comprise secondthermoplastic resin, and/or the first thermoplastic resin may differfrom the second thermoplastic resin.

In some examples, a stiffening element that comprises one or moreintegral current flowpaths may comprise: a first layer comprising afirst aluminum layer; a second layer, adjacent to the first layer,comprising one or more first glass-fiber-reinforced thermoplastic plies;a third layer, adjacent to the second layer and opposite to the firstlayer, comprising a plurality of carbon-fiber-reinforced thermoplasticplies; a fourth layer, adjacent to the third layer and opposite to thesecond layer, comprising one or more second glass-fiber-reinforcedthermoplastic plies; and/or a fifth layer, adjacent to the fourth layerand opposite to the third layer, comprising a second aluminum layer. Thefifth layer may be configured to form a first outer surface of thestiffening element. The fifth layer may be configured to form at leastpart of the one or more integral current flowpaths.

In some examples, the first layer may be configured to form a secondouter surface of the stiffening element.

In some examples, the first and second aluminum layers may comprise asame aluminum alloy.

In some examples, the first and second aluminum layers may comprisedifferent aluminum alloys.

In some examples, the first layer may be configured to form at leastpart of the one or more integral current flowpaths.

In some examples, the first layer may be configured to form at leastpart of a first flowpath of the one or more integral current flowpaths,the fifth layer may be configured to form at least part of a secondflowpath of the one or more integral current flowpaths, and/or the firstflowpath may differ from the second flowpath.

In some examples, current flow in the first flowpath may besubstantially parallel to and in a same direction as current flow in thesecond flowpath.

In some examples, current flow in the first flowpath may besubstantially parallel to but in an opposite direction from current flowin the second flowpath.

In some examples, a method of using a stiffening element that comprisesone or more integral current flowpaths as part of a current returnnetwork for a stiffened structure may comprise: selecting the stiffeningelement comprising a first layer that comprises a first aluminum layer,a second layer adjacent to the first layer that comprises one or morefirst glass-fiber-reinforced thermoplastic plies, a third layer adjacentto the second layer and opposite to the first layer that comprises aplurality of carbon-fiber-reinforced thermoplastic plies, a fourth layeradjacent to the third layer and opposite to the second layer thatcomprises one or more second glass-fiber-reinforced thermoplastic plies,a fifth layer adjacent to the fourth layer and opposite to the thirdlayer that comprises a second aluminum layer, wherein the fifth layer isconfigured to form a first outer surface of the stiffening element, andwherein the fifth layer is configured to form at least part of the oneor more integral current flowpaths; and/or routing current from thecurrent return network through the one or more integral currentflowpaths of the selected stiffening element.

In some examples, the routing of the current from the current returnnetwork may comprise routing the current from the current return networkthrough the fifth layer.

In some examples, the stiffening element may comprise first and secondintegral current flowpaths, and/or the routing of the current from thecurrent return network may comprise routing the current from the currentreturn network through the first and second integral current flowpaths.

In some examples, current flow in the first integral current flowpathmay be substantially parallel to and in a same direction as current flowin the second integral current flowpath, or the current flow in thefirst integral current flowpath may be substantially parallel to but inan opposite direction from the current flow in the second integralcurrent flowpath.

In some examples, a stiffening element that comprises one or moreintegral current flowpaths may comprise: a first layer comprising aplurality of carbon-fiber-reinforced thermoplastic plies configured toform a first surface and a second surface, where the first surface isopposite to the second surface; a second layer, adjacent to the firstsurface, comprising one or more first glass-fiber-reinforcedthermoplastic plies; a third layer, adjacent to the second surface,comprising one or more second glass-fiber-reinforced thermoplasticplies; a fourth layer, adjacent to the second layer and opposite to thefirst layer, comprising a first aluminum layer; and/or a fifth layer,adjacent to the third layer and opposite to the first layer, comprisinga second aluminum layer. The fourth layer may be configured to form afirst outer surface of the stiffening element. The fourth layer may beconfigured to form at least part of the one or more integral currentflowpaths.

In some examples, the fifth layer may be configured to form a secondouter surface of the stiffening element.

In some examples, the first and second aluminum layers may comprise asame aluminum alloy.

In some examples, the first and second aluminum layers may comprisedifferent aluminum alloys.

In some examples, the fifth layer may be configured to form at leastpart of the one or more integral current flowpaths.

In some examples, the fourth layer may be configured to form at leastpart of a first flowpath of the one or more integral current flowpaths,the fifth layer may be configured to form at least part of a secondflowpath of the one or more integral current flowpaths, and/or the firstflowpath may be different from the second flowpath.

In some examples, current flow in the first flowpath may besubstantially parallel to and in a same direction as current flow in thesecond flowpath.

In some examples, current flow in the first flowpath may besubstantially parallel to but in an opposite direction from current flowin the second flowpath.

In some examples, a method of using a stiffening element that comprisesone or more integral current flowpaths as part of a current returnnetwork for a stiffened structure may comprise: selecting the stiffeningelement comprising a first layer that comprises a plurality ofcarbon-fiber-reinforced thermoplastic plies configured to form a firstsurface and a second surface where the first surface is opposite to thesecond surface, a second layer adjacent to the first surface thatcomprises one or more first glass-fiber-reinforced thermoplastic plies,a third layer adjacent to the second surface that comprises one or moresecond glass-fiber-reinforced thermoplastic plies, a fourth layeradjacent to the second layer and opposite to the first layer thatcomprises a first aluminum layer, and a fifth layer adjacent to thethird layer and opposite to the first layer that comprises a secondaluminum layer, wherein the fourth layer is configured to form a firstouter surface of the stiffening element, and wherein the fourth layer isconfigured to form at least part of the one or more integral currentflowpaths; and/or routing current from the current return networkthrough the one or more integral current flowpaths of the selectedstiffening element.

In some examples, the routing of the current from the current returnnetwork may comprise routing the current from the current return networkthrough the fourth layer.

In some examples, the stiffening element may comprise first and secondintegral current flowpaths, and/or the routing of the current from thecurrent return network may comprise routing the current from the currentreturn network through the first and second integral current flowpaths.

In some examples, current flow in the first integral current flowpathmay be substantially parallel to and in a same direction as current flowin the second integral current flowpath, or the current flow in thefirst integral current flowpath may be substantially parallel to but inan opposite direction from the current flow in the second integralcurrent flowpath.

In some examples, a method of using a stiffening element that comprisesone or more integral current flowpaths as part of a current returnnetwork for a stiffened structure may comprise: selecting the stiffeningelement comprising a first layer comprising a plurality ofcarbon-fiber-reinforced thermoplastic plies, a second layer adjacent tothe first layer comprising one or more glass-fiber-reinforcedthermoplastic plies, and a third layer adjacent to the second layer andopposite to the first layer comprising aluminum, wherein the third layeris configured to form an outer surface of the stiffening element, andwherein the third layer is configured to form at least part of the oneor more integral current flowpaths; and/or routing current from thecurrent return network through the one or more integral currentflowpaths of the selected stiffening element.

In some examples, the routing of the current from the current returnnetwork may comprise routing the current from the current return networkthrough the third layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the present teachings, as claimed.

DRAWINGS

The above and/or other aspects and advantages will become more apparentand more readily appreciated from the following detailed description ofexamples, taken in conjunction with the accompanying drawings, in which:

FIG. 1A shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 1B shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 1C shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 1D shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 1E shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 2A shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 2B shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 2C shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 2D shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 2E shows a stiffening element, according to some examples of thedisclosed stiffening elements;

FIG. 3A shows laying up a first layer on a mold tool, according to someexamples of the disclosed stiffening elements;

FIG. 3B shows laying up a second layer on the first layer of FIG. 3A,according to some examples of the disclosed stiffening elements;

FIG. 3C shows laying up a third layer on the second layer of FIG. 3B,according to some examples of the disclosed stiffening elements;

FIG. 3D shows a stack comprising a first carbon composite layer, a firstglass composite layer, a second carbon composite layer, a second glasscomposite layer, and/or an aluminum layer, according to some examples ofthe disclosed stiffening elements;

FIG. 4A shows a temperature versus time profile for consolidating one ormore thermoplastic prepreg plies at a temperature sufficient to softenan aluminum layer, or for adjusting the temperature and pressure of astack, according to some examples of the disclosed stiffening elements;

FIG. 4B shows a pressure versus time profile for consolidating one ormore thermoplastic prepreg plies at a temperature sufficient to softenan aluminum layer, or for adjusting the temperature and pressure of astack, according to some examples of the disclosed stiffening elements;

FIG. 4C shows a temperature and pressure versus time profile forconsolidating one or more thermoplastic prepreg plies at a temperaturesufficient to soften an aluminum layer, or for adjusting the temperatureand pressure of a stack, according to some examples of the disclosedstiffening elements; and

FIG. 5 shows a stiffening element, according to some examples of thedisclosed stiffening elements.

DETAILED DESCRIPTION

Exemplary aspects will now be described more fully with reference to theaccompanying drawings. Examples of the disclosure, however, may beembodied in many different forms and should not be construed as beinglimited to the examples set forth herein. Rather, these examples areprovided so that this disclosure will be thorough and complete, and willfully convey the scope to those skilled in the art. In the drawings,some details may be simplified and/or may be drawn to facilitateunderstanding rather than to maintain strict structural accuracy,detail, and/or scale. For example, the thicknesses of layers and regionsmay be exaggerated for clarity.

It will be understood that when an element is referred to as being “on,”“connected to,” “electrically connected to,” or “coupled to” to anothercomponent, it may be directly on, connected to, electrically connectedto, or coupled to the other component or intervening components may bepresent. In contrast, when a component is referred to as being “directlyon,” “directly connected to,” “directly electrically connected to,” or“directly coupled to” another component, there are no interveningcomponents present. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

It will be understood that although the terms first, second, third,etc., may be used herein to describe various elements, components,regions, layers, and/or sections, these elements, components, regions,layers, and/or sections should not be limited by these terms. Theseterms are only used to distinguish one element, component, region,layer, and/or section from another element, component, region, layer,and/or section. For example, a first element, component, region, layer,or section could be termed a second element, component, region, layer,or section without departing from the teachings of examples.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like may be used herein for ease of description todescribe the relationship of one component and/or feature to anothercomponent and/or feature, or other component(s) and/or feature(s), asillustrated in the drawings. It will be understood that the spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation(s) depicted inthe figures.

The terminology used herein is for the purpose of describing particularexamples only and is not intended to be limiting of examples. As usedherein, the singular forms “a,” “an,” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which examples belong. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andshould not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

As discussed above, many industries continually seek to push theboundaries of what has come before in structures such as aircraft,bridges, buildings, cars, locomotives, missiles, rockets, ships,stiffening elements, submarines, submersibles, towers, train cars, andtrucks. In particular, as cost and weight reduction may be primaryfactors, relatively inexpensive, lighter composite materials may besubstituted for relatively more expensive, heavier metals in thosestructures. The relatively lighter composite materials may be equallystrong or even stronger than the relatively heavier metals, but thecomposite materials may not be able to be used in the same manner as themetals, particularly with regard to electrical functions.

In response to these and related concerns, some industries (e.g.,aerospace) may incorporate additional current flowpaths into theirproducts (e.g., aircraft). For example, concerns due to the substitutionof composite materials for conductive aluminum and other metallicstructures in an aircraft may be addressed using a current returnnetwork (“CRN”). Such a CRN, for example, may internally connect variouspreexisting metallic structures distributed in the aircraft with wiringso as to provide, for example, EME conductivity (e.g., protection fromhigh-intensity radiated fields (“HIRE”), lightning-strike protection(“LSP”)) as part of an airframe electrical grounding system (e.g.,metallic ground plane), although potentially adding weight andcomplexity. Such a CRN may include multiple paths, connected inparallel, to provide redundant current flowpaths. The multiple paths maybe separated within the aircraft to provide additional protectionagainst internal or external threats (e.g., upper crown and lower bilgearea of main fuselage, leading and trailing edges of wings).

Portions of such CRNs may be fabricated, for example, with aluminumextrusions. Such aluminum extrusions may be made withnon-structural-grade aluminum having relatively low resistivity values,as opposed to relatively higher resistivity values often associated withstructural-grade aluminum. However, the aluminum extrusions made withnon-structural-grade aluminum may not be as structurally efficient andstiff (e.g., self-supporting) as some traditional composite materials.But traditional composite materials may not be as electricallyconductive as such aluminum extrusions made with non-structural-gradealuminum.

In response to these and related concerns in the aerospace industry andother industries, the present disclosure is directed to stiffeningelements. Such stiffening elements may be lightweight, strong, andcorrosion resistant, and may comprise one or more integral currentreturn flowpaths. An overall modulus of elasticity for the stiffeningelements, for example, may be greater than or equal to 5×10⁶ pounds persquare inch and less than or equal to 20×10⁶ pounds per square inch.Non-rigid joints and attachments (e.g., fasteners, splice plates,collared bolts) for such stiffening elements, and conductivity betweensections (e.g., electrical jumper straps), may accommodate thermalexpansion and flexure.

As known to one of ordinary skill in the art, traditional CRNs maycomprise, for example, a combination of systems structures, aluminumstructures, titanium structures, and/or dedicated CRN components. Thedisclosed stiffening elements may replace at least such dedicated CRNcomponents and/or such aluminum structures. In addition or in thealternative, the disclosed stiffening elements may be used to render aCRN independent of such systems structures and/or titanium structures.

FIG. 1A shows stiffening element 100A, according to some examples of thedisclosed stiffening elements. As shown in FIG. 1A, stiffening element100A comprises: aluminum layer 102A; glass composite layer 104A adjacentto aluminum layer 102A; and carbon composite layer 106A adjacent toglass composite layer 104A, and opposite to aluminum layer 102A.

Glass composite layer 104A may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. Carbon compositelayer 106A may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies.

As used herein, the term “prepreg” is an abbreviation for“pre-impregnated” composite fibers in which a matrix material, such asthermoplastic resin, is already present in the fiber reinforcementbefore molding occurs. Prepreg manufacturing techniques may be employedto manufacture composite parts for a variety of commercial usesincluding, for example, the manufacture of aircraft and/or spacecraft.

As used herein, the term “prepreg plies” includes both prepreg fabricsand prepreg tapes.

Stiffening element 100A may provide, for example, reduced weight pergiven volume as compared to simple metal structures.

Aluminum layer 102A may be configured to form the outer surface ofstiffening element 100A or an outer surface of stiffening element 100A.Heat transfer via conduction, convection, and/or radiation near and/orat outer surfaces of a stiffening element (e.g., stiffening element100A) may limit potential heating damage to the stiffening element.

Stiffening element 100A may comprise one or more integral currentflowpaths. Aluminum layer 102A may be configured to form at least partof the one or more integral current flowpaths.

As used herein, the term “integral current flowpath” means that thecurrent flowpath is provided by stiffening element 100A itself, and isnot reliant upon other components, particularly other metalliccomponents that might add weight to stiffening element 100A (e.g., addedcopper foil or mesh). The EME conductivity also is not reliant upon, forexample, ply-integrated interwoven wires (e.g., interwoven wire fabric),conductive nonwoven veils, conductive paints, or conductive surfacingfilms.

Aluminum layer 102A, glass composite layer 104A, and carbon compositelayer 106A may have the same or different thicknesses.

Aluminum layer 102A may or may not have aluminum sublayers; glasscomposite layer 104A may or may not have glass composite sublayers;and/or carbon composite layer 106A may or may not have carbon compositesublayers.

As used herein, the term “aluminum” means the metallic element of atomicnumber 13, including any isotopes thereof.

As used herein, the term “alloy” means a solid or liquid mixture of twoor more metals, or of one or more metals with one or more nonmetallicelements, as in carbon steels.

As used herein, the term “layer” means a thickness of material laid on,formed on, or spread over a surface, body, or portion of a surface orbody. A layer may cover the surface, body, or portion of the surface orbody, or form an overlying part or segment of material that covers thesurface, body, or portion of the surface or body. A layer may haveconstant or variable thickness.

As used herein, the term “aluminum layer” means a layer comprisingaluminum. An aluminum layer may comprise, for example, pure aluminum, analuminum alloy, or some other substance that comprises aluminum. Thealuminum layer may comprise, for example, 1100 series aluminum (e.g., acommercially pure alloy of aluminum, such as 1100-O, 1100-H12, 1100-H14,1100-H16, 1100-H18, 1100-H22, 1100-H24, 1100-H26, 1100-H28, 1100-H112,or 1100-H113 aluminum according to the International Alloy DesignationSystem (“IADS”)). More generally, the aluminum layer may comprise, forexample, a 1000 series aluminum alloy (e.g., aluminum alloy 1050, 1060,1100, 1145, 1199, 1200, 1230, or 1350 according to IADS); a 2000 seriesaluminum alloy (e.g., aluminum alloy 2008, 2011, 2014, 2017, 2018, 2024,2025, 2036, 2048, 2090, 2117, 2124, 2127, 2195, 2218, 2219, 2224, 2319,2324, 2524, or 2618 according to IADS); a 3000 series aluminum alloy(e.g., aluminum alloy 3003, 3004, 3005, 3102, or 3105 according toIADS); a 5000 series aluminum alloy (e.g., aluminum alloy 5005, 5050,5052, 5056, 5059, 5083, 5086, 5154, 5182, 5183, 5252, 5254, 5356, 5357,5454, 5456, 5457, 5554, 5556, 5652, 5654, 5657, or 5754 according toIADS); a 6000 series aluminum alloy (e.g., aluminum alloy 6003, 6005,6005A, 6009, 6010, 6013, 6016, 6053, 6060, 6061, 6062, 6063, 6066, 6070,6082, 6101 (e.g., 6101-H111, 6101-T64), 6105, 6111, 6151, 6162, 6201,6205, 6253, 6262, 6351, 6463, or 6951 according to IADS); or a 7000series aluminum alloy (e.g., aluminum alloy 7001, 7005, 7008, 7022,7039, 7049, 7050, 7055, 7068, 7072, 7075, 7076, 7079, 7108, 7116, 7129,7150, 7175, 7178, or 7475 according to IADS).

The pure aluminum, aluminum alloys, and/or other substances thatcomprise aluminum discussed above may be compatible withhigh-temperature processing (e.g., at temperatures required forthermoplastic processing or consolidation, such as ≥600° F., ≥650° F.,≥675° F., or ≥700° F., but less than the melting temperature of the purealuminum, aluminum alloys, and/or other substances that comprisealuminum). For example, the pure aluminum, aluminum alloys, and/or othersubstances that comprise aluminum discussed above may exhibit low yieldstrength(s) (e.g., ≤7.2×10³ pounds per square inch gage (“psig”),≤5.8×10³ psig, ≤5.1×10³ psig, or ≤4.3×10³ psig), helping to reduceresidual thermal stresses in other layers during and/or after cooldown.Other factors may help to reduce residual thermal stresses in otherlayers during and/or after cooldown, such as the thickness of the purealuminum, aluminum alloys, and/or other substances that comprisealuminum discussed above; the existence and number of sublayers of thepure aluminum, aluminum alloys, and/or other substances that comprisealuminum discussed above; and/or whether the pure aluminum, aluminumalloys, and/or other substances that comprise aluminum discussed aboveare in direct contact with a specific adjacent layer (e.g., a glasscomposite layer).

The pure aluminum, aluminum alloys, and/or other substances thatcomprise aluminum discussed above may exhibit high electricalconductivity (e.g., ≥45% per the International Annealed Copper Standard(“IACS”), ≥50% IACS, ≥55% IACS, or ≥57% IACS), helping to conductelectricity (e.g., power returns, steady state current returns, faultcurrent returns) and/or to provide integral electromagnetic effects(“EME”) conductivity (e.g., protection from high-intensity radiatedfields (“HIRF”), lightning-strike protection (“LSP”)), for example, inweight-efficient composite aircraft structures. Other factors may helpto conduct electricity and/or to provide integral EME conductivity, suchas the thickness of the pure aluminum, aluminum alloys, and/or othersubstances that comprise aluminum discussed above; the existence andnumber of sublayers of the pure aluminum, aluminum alloys, and/or othersubstances that comprise aluminum discussed above; and/or whether thepure aluminum, aluminum alloys, and/or other substances that comprisealuminum discussed above are in direct contact with an airframeelectrical grounding system (e.g., metallic ground plane).

In addition to conductivity and resistivity, other material properties(e.g., corrosion resistance) and physical dimensions (e.g.,cross-sectional area) may be important in ensuring that stiffeningelements have the required level of current handling capability.

As used herein, the term “integral EME conductivity” means that the EMEconductivity is provided by stiffening element 100A itself, and is notreliant upon other components, particularly other metallic componentsthat might add weight to stiffening element 100A (e.g., added copperfoil or mesh). The EME conductivity also is not reliant upon, forexample, ply-integrated interwoven wires (e.g., interwoven wire fabric),conductive nonwoven veils, conductive paints, or conductive surfacingfilms.

The pure aluminum, aluminum alloys, and/or other substances thatcomprise aluminum discussed above may exhibit high thermal conductivity(e.g., ≥70 British thermal units/hour-foot-degree Fahrenheit(“BTU/hr-ft-° F.”), ≥85 BTU/hr-ft-° F., ≥100 BTU/hr-ft-° F., ≥115BTU/hr-ft-° F., or ≥130 BTU/hr-ft-° F., helping to transfer, dissipate,and/or distribute thermal energy of the stiffening elements. Otherfactors may help to transfer, dissipate, and/or distribute thermalenergy, such as the thickness of the pure aluminum, aluminum alloys,and/or other substances that comprise aluminum discussed above; theexistence and number of sublayers of the pure aluminum, aluminum alloys,and/or other substances that comprise aluminum discussed above; and/orwhether the pure aluminum, aluminum alloys, and/or other substances thatcomprise aluminum discussed above are in direct contact with a specificadjacent layer and/or an airframe electrical grounding system (e.g.,metallic ground plane).

Surfaces of the aluminum layer may undergo surface preparation, such asalkaline degreasing, chromic acid anodizing or other anodizingprocessing, priming (e.g., with BR 127 corrosion-inhibiting primer),sol-gel, and/or pickling in chromic-sulfuric acid. The surfaces also maybe roughened, for example, by abrasion. Such surface preparation mayenhance bonding between the aluminum layer(s) and other layers.

The aluminum layer(s) may be depended on to provide current-carryingcapacity for the stiffening element. Generally, in such cases, thenumber and/or thickness of the aluminum layer(s) are greater than whenthe aluminum layer(s) are not depended on to provide current-carryingcapacity.

The aluminum layer(s) may be depended on to provide integral EMEconductivity for the stiffening element. Generally, in such cases, thenumber and/or thickness of the aluminum layer(s) are greater than whenthe aluminum layer(s) are not depended on to provide integral EMEconductivity.

The aluminum layer(s) may be depended on to provide thermal conductivityfor the stiffening element. For example, the thermal conductivity mayassist in a heat sink function via a CRN. Generally, in such cases, thenumber and/or thickness of the aluminum layer(s) are greater than whenthe aluminum layer(s) are not depended on to provide thermalconductivity.

The aluminum layer(s) may be depended on to provide significantstructural support. Generally, in such cases, the number and/orthickness of the aluminum layer(s) are greater than when the aluminumlayer(s) are not depended on to provide significant structural support.Whether or not depended on to provide significant structural support,thicknesses of the aluminum layer(s) may be, for example, ≥0.005 inchesand ≤0.020 inches (e.g., 0.005 inches, 0.010 inches, 0.015 inches, or0.020 inches). When depended on to provide significant structuralsupport, thicknesses of the aluminum layer(s) may be even greater than0.0020 inches.

As used herein, the term “composite” means a mixture or mechanicalcombination on a macroscale of two or more materials that are solid inthe finished state, are mutually insoluble, and differ in chemicalnature.

As used herein, the term “tenacity” means the strength per unit weightof a fiber, typically expressed in grams per denier.

As used herein, the term “fiber” means a fundamental form of solid(usually crystalline) characterized by relatively high tenacity and anextremely high ratio of length to diameter (e.g., several hundred ormore to one). Semisynthetic fibers include inorganic substances extrudedin fibrous form using, for example, carbon or glass. Synthetic fibersinclude substances extruded in fibrous form using, for example, highpolymers.

As used herein, the term “glass” means a non-crystalline, amorphoussolid. The glass may comprise, for example, a ceramic materialcomprising a mixture of silica, soda ash, and lime. The glass maycomprise, for example, one or more of C-glass, E-glass, S-glass, orT-glass. The glass may be, for example, in the form of glass fibers(e.g., fiberglass). The glass may comprise, for example, S-2 glass(e.g., S-2 glass fibers).

The glass fibers may be woven or nonwoven (e.g., chopped, matted, orrandomly oriented). The strength of the woven fibers may vary with thetype of weave and/or the orientation of the woven fibers (e.g., if thewoven fibers are oriented in parallel, the strength of the woven fibersas a group should be greater in directions parallel to thatorientation). The type of weave may be, for example, a plain weave(e.g., 1×1), a twill weave (e.g., 2×2), a basket weave, a fish weave, aharness weave, a leno weave, a satin weave, or a unidirectional weave.

As used herein, the term “matrix” means a substance used to holdtogether strength members of a composite, where the substance is one ofthe two or more materials of the composite.

As used herein, the term “resin” means a semisolid or solid complexamorphous mix of organic compounds.

As used herein, the term “monomer” means a molecule or compound, usuallycomprising carbon, and of relatively low molecular weight and simplestructure.

As used herein, the term “polymer” means a macromolecule formed by thechemical union of five or more identical monomers. A polymer may be, forexample, inorganic or organic. An organic polymer may be, for example,natural or synthetic (e.g., man-made). A synthetic organic polymer maybe, for example, thermoplastic or thermosetting.

As used herein, the term “high polymer” means an organic polymer havinga molecular weight ≥5,000 grams/mole.

As used herein, the term “thermoplastic” means a high polymer, asdefined above, that softens when exposed to heat and returns to itsoriginal condition when cooled to room temperature. A thermoplasticpolymer may be, for example, amorphous or semi-crystalline. Athermoplastic polymer may comprise, for example, one or more ofpolyaryletherketone (“PAEK”), polyetherimide (“PEI”), or polyphenylenesulfide (“PPS”). A polyaryletherketone may comprise, for example, one ormore of polyetherketone (“PEK”), polyetheretherketone (“PEEK”),polyetherketoneketone (“PEKK”), polyetheretherketoneketone (“PEEKK”), orpolyetherketoneetherketoneketone (“PEKEKK”).

As used herein, the term “thermosetting polymer” means a high polymer,as defined above, that crosslinks upon the application of heat, andsolidifies or “sets” irreversibly.

As used herein, the term “glass composite layer” means a layercomprising a composite that comprises glass. The glass may be, forexample, in the form of glass fibers. The glass fibers in the glasscomposite layer may have no specific orientation (e.g., omnidirectional)or may be oriented in one or more directions (e.g., unidirectional,bidirectional, or multidirectional). The glass fibers may be aligned,continuous, and/or unidirectional.

A glass composite layer comprises, for example, a matrix. The matrix maycomprise, for example, resin. The resin may comprise, for example, athermoplastic polymer. The thermoplastic polymer may comprise, forexample, one or more of PEEK (PEEK has a relatively high glasstransition temperature (about 290° F.) and melting temperature (about650° F.), allowing for high-temperature processing), PEKK (PEKK has arelatively high glass transition temperature (about 315° F.) and meltingtemperature (about 640° F.), allowing for high-temperature processing),or other thermoplastic polymers. A glass composite layer may comprise,for example, glass-fiber-reinforced polymer(s). A glass composite layermay comprise, for example, glass-fiber-reinforced thermoplasticpolymer(s).

The thermoplastic resin of the one or more glass-fiber-reinforcedthermoplastic prepreg plies provides binding for the glass fibers. Thethermoplastic resin may exhibit a sufficiently high glass transitiontemperature, continuous service temperature, and/or crystallite meltingpoint so as to allow the aluminum layer(s) (e.g., aluminum layer 102A)to be softened for molding, shaping, and/or other processes associatedwith manufacture of the stiffening element(s).

The glass composite layer(s) may be depended on to provide significantstructural support. Generally, in such cases, the number and/orthickness of the glass composite layer(s) are greater than when theglass composite layer(s) are not depended on to provide significantstructural support.

Whether or not depended on to provide significant structural support,thicknesses of the glass composite layer(s) may be, for example, ≥0.0020inches and ≤0.0080 inches (e.g., 0.0020 inches, 0.0025 inches, 0.0030inches, 0.0035 inches, 0.0040 inches, 0.0045 inches, 0.0050 inches,0.0055 inches, 0.0060 inches, 0.0065 inches, 0.0070 inches, 0.0075inches, or 0.0080 inches). When depended on to provide significantstructural support, thicknesses of the glass composite layer(s) may beeven greater than 0.0080 inches.

Whether or not depended on to provide significant structural support,thicknesses of the sublayers of the glass composite layer(s) may be, forexample, ≥0.0020 inches and ≤0.0040 inches (e.g., 0.0020 inches, 0.0025inches, 0.0030 inches, 0.0035 inches, or 0.0040 inches). When dependedon to provide significant structural support, thicknesses of the glasscomposite sublayer(s) may be even greater than 0.0040 inches.

As used herein, the term “adjacent” means “near or directly contacting.”

Resin of the glass composite layer may directly bond the aluminum layerand the glass composite layer (e.g., aluminum layer 102A and glasscomposite layer 104A). In such cases, glass composite layer 104A may beadjacent to aluminum layer 102A, and may directly contact aluminum layer102A.

An additional layer (not shown) may be between aluminum layer 102A andglass composite layer 104A. In such cases, glass composite layer 104Amay be adjacent to aluminum layer 102A, but may not directly contactaluminum layer 102A. The additional layer may improve the bonding ofaluminum layer 102A and glass composite layer 104A. The additional layermay at least partially decouple effects (e.g., thermal contraction,thermal expansion, strains, or stresses) associated with the bonding ofaluminum layer 102A and glass composite layer 104A.

The additional layer may be, for example, an adhesive layer. Care shouldbe taken during selection of material(s) for such an additional layerbecause, for example, some adhesives comprise silver or other elementsor compounds that may interact with aluminum layer 102A and/or glasscomposite layer 104A via one or more interaction mechanisms (e.g.,galvanic corrosion).

As used herein, the term “carbon” means the nonmetallic element ofatomic number 6, including any isotopes thereof.

As used herein, the term “carbon composite layer” means a layercomprising a composite that comprises carbon. The carbon may be, forexample, in the form of carbon fibers. The carbon fibers in the carboncomposite layer may have no specific orientation (e.g., omnidirectional)or may be oriented in one or more directions (e.g., unidirectional,bidirectional, or multidirectional). The carbon fibers may be aligned,continuous, and/or unidirectional.

The carbon fibers may be woven. The strength of the woven fibers mayvary with the type of weave and/or the orientation of the woven fibers(e.g., if the woven fibers are oriented in parallel, the strength of thewoven fibers as a group should be greater in directions parallel to thatorientation). The type of weave may be, for example, a plain weave(e.g., 1×1), a twill weave (e.g., 2×2), a basket weave, a fish weave, aharness weave, a leno weave, a satin weave, or a unidirectional weave.

A carbon composite layer comprises, for example, a matrix. The matrixmay comprise, for example, resin. The resin may comprise, for example, athermoplastic polymer. The thermoplastic polymer may comprise, forexample, one or more of PEEK, PEKK, or other thermoplastic polymers. Acarbon composite layer may comprise, for example,carbon-fiber-reinforced polymer(s). A carbon composite layer maycomprise, for example, carbon-fiber-reinforced thermoplastic polymer(s).

The thermoplastic resin of the one or more carbon-fiber-reinforcedthermoplastic prepreg plies provides binding for the carbon fibers. Thethermoplastic resin may exhibit a sufficiently high glass transitiontemperature, continuous service temperature, and/or crystallite meltingpoint so as to allow the aluminum layer(s) (e.g., aluminum layer 102A)to be softened for molding, shaping, and/or other processes associatedwith manufacture of the stiffening element(s).

The carbon composite layer(s) may be depended on to provide significantstructural support. Generally, in such cases, the number and/orthickness of the carbon composite layer(s) are greater than when thecarbon composite layer(s) are not depended on to provide significantstructural support.

Whether or not depended on to provide significant structural support,thicknesses of the carbon composite layer(s) may be, for example,≥0.0400 inches and ≤0.1000 inches (e.g., 0.0400 inches, 0.0432 inches,0.0440 inches, 0.0450 inches, 0.0500 inches, 0.0550 inches, 0.0600inches, 0.0650 inches, 0.0700 inches, 0.0750 inches, 0.0800 inches,0.0850 inches, 0.0900 inches, 0.0950 inches, 0.1000 inches). Whendepended on to provide significant structural support, thicknesses ofthe carbon composite layer(s) may be even greater than 0.0800 inches.

Whether or not depended on to provide significant structural support,thicknesses of the sublayers of the carbon composite layer(s) may be,for example, ≥0.0040 inches and 0.0080 inches (e.g., 0.0040 inches,0.0044 inches, 0.0045 inches, 0.0050 inches, 0.0054 inches, 0.0055inches, 0.0060 inches, 0.0065 inches, 0.0070 inches, 0.0075 inches, or0.0080 inches). When depended on to provide significant structuralsupport, thicknesses of the carbon composite sublayer(s) may be evengreater than 0.0080 inches.

The one or more glass-fiber-reinforced thermoplastic prepreg plies maycomprise first thermoplastic resin, the one or morecarbon-fiber-reinforced thermoplastic prepreg plies may comprise secondthermoplastic resin, and the first thermoplastic resin may be the sameas the second thermoplastic resin. When the first thermoplastic resin isthe same as the second thermoplastic resin, manufacture of thestiffening element(s) may be simplified, and the bonding between the oneor more glass-fiber-reinforced thermoplastic prepreg plies and the oneor more carbon-fiber-reinforced thermoplastic prepreg plies may be moreuniform and/or more stable over time due, for example, to compatibilityof the first and second thermoplastic resins.

The one or more glass-fiber-reinforced thermoplastic prepreg plies maycomprise first thermoplastic resin, the one or morecarbon-fiber-reinforced thermoplastic prepreg plies may comprise secondthermoplastic resin, and the first thermoplastic resin may differ fromthe second thermoplastic resin. When the first thermoplastic resindiffers from the second thermoplastic resin, more design options may beavailable during manufacture of the stiffening element(s), and thebonding between the one or more glass-fiber-reinforced thermoplasticprepreg plies and the one or more carbon-fiber-reinforced thermoplasticprepreg plies may be stronger initially and/or over time, particularlyif the first and second thermoplastic resins are selected for mutualchemical compatibility.

The glass composite layer(s) may be configured to prevent interaction(e.g., direct) between the aluminum layer and the carbon composite layer(e.g., glass composite layer 104A may be configured to preventinteraction between aluminum layer 102A and carbon composite layer106A). The glass composite layer(s) may be configured to preventgalvanic corrosion due to interaction (e.g., direct or indirect) betweenthe aluminum layer and the carbon composite layer (e.g., glass compositelayer 104A may be configured to prevent galvanic corrosion due tointeraction between aluminum layer 102A and carbon composite layer106A).

The glass composite layer(s) may be configured to reduce thermal stress,during cooldown (e.g., during a consolidation process), due todifferences in thermal contraction between the aluminum layer and thecarbon composite layer (e.g., glass composite layer 104A may beconfigured to reduce thermal stress, during cooldown, due to differencesin thermal contraction between aluminum layer 102A and carbon compositelayer 106A), for example, by at least partially decoupling the effectsof thermal contraction in the aluminum and carbon composite layersand/or through effects associated with orientation of glass fibers inthe glass composite layer(s). The glass composite layer(s) may beconfigured to reduce residual thermal stress, after cooldown (e.g.,after a consolidation process), due to differences in thermalcontraction, during cooldown, between the aluminum layer and the carboncomposite layer (e.g., glass composite layer 104A may be configured toreduce residual thermal stress, after cooldown, due to differences inthermal contraction, during cooldown, between aluminum layer 102A andcarbon composite layer 106A), for example, by at least partiallydecoupling the effects of thermal contraction in the aluminum and carboncomposite layers and/or through effects associated with orientation ofglass fibers in the glass composite layer(s).

One or more glass composite layers may be configured to reduce thermalstress, both during and after cooldown, functioning as a compliant layeror layers. This compliant functioning helps to avoid separation ofadjacent layers due to the build-up of stress near, at, or acrossboundaries between the adjacent layers.

Resin of the glass composite layer may directly bond the glass compositelayer and the carbon composite layer (e.g., glass composite layer 104Aand carbon composite layer 106A). In such cases, glass composite layer104A may be adjacent to carbon composite layer 106A, and may directlycontact carbon composite layer 106A.

Resin of the carbon composite layer may directly bond the glasscomposite layer and the carbon composite layer (e.g., glass compositelayer 104A and carbon composite layer 106A). In such cases, glasscomposite layer 104A may be adjacent to carbon composite layer 106A, andmay directly contact carbon composite layer 106A.

Resins of the glass and carbon composite layers may directly bond theglass composite layer and the carbon composite layer (e.g., glasscomposite layer 104A and carbon composite layer 106A). In such cases,glass composite layer 104A may be adjacent to carbon composite layer106A, and may directly contact carbon composite layer 106A.

An additional layer (not shown) may be between glass composite layer104A and carbon composite layer 106A. In such cases, glass compositelayer 104A may be adjacent to carbon composite layer 106A, but may notdirectly contact carbon composite layer 106A. The additional layer mayimprove the bonding of glass composite layer 104A and carbon compositelayer 106A. The additional layer may at least partially decouple effects(e.g., thermal contraction, thermal expansion, strains, or stresses)associated with the bonding of glass composite layer 104A and carboncomposite layer 106A.

The additional layer may be, for example, an adhesive layer. Care shouldbe taken during selection of material(s) for such an additional layerbecause, for example, some adhesives comprise elements or compounds thatmay interact with glass composite layer 104A and/or carbon compositelayer 106A via one or more interaction mechanisms.

One or more layers may be between a given aluminum layer and a givencarbon composite layer. The one or more layers may comprise, forexample, glass composite layer(s) and/or adhesive layer(s). The one ormore layers may be configured to prevent interaction (e.g., direct)between the aluminum layer(s) and the carbon composite layer(s) (e.g.,configured to prevent interaction between aluminum layer 102A and carboncomposite layer 106A). The one or more layers may be configured toprevent galvanic corrosion due to interaction (e.g., direct or indirect)between the aluminum layer(s) and the carbon composite layer(s) (e.g.,configured to prevent galvanic corrosion due to interaction betweenaluminum layer 102A and carbon composite layer 106A).

FIG. 1B shows stiffening element 100B, according to some examples of thedisclosed stiffening elements. As shown in FIG. 1B, stiffening element100B comprises: aluminum layer 102B; glass composite layer 104B adjacentto aluminum layer 102B; and carbon composite layer 106B adjacent toglass composite layer 104B, and opposite to aluminum layer 102B.

Glass composite layer 104B may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. Carbon compositelayer 106B may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies.

Aluminum layer 102B may comprise a plurality of aluminum sublayers102B1, 102B2, 102B3, 102B4 (e.g., 102B1, 102B2, . . . , 102Bn). Thenumber (n) of aluminum sublayers may be, for example, 2, 3, 4, 5, 6, ormore sublayers.

Aluminum layer 102B, glass composite layer 104B, and carbon compositelayer 106B may have the same or different thicknesses. Similarly,plurality of aluminum sublayers 102B1, 102B2, 102B3, 102B4 may have thesame or different thicknesses.

Aluminum layer 102B (e.g., aluminum sublayer 102B1) may be configured toform the outer surface of stiffening element 100B or an outer surface ofstiffening element 100B.

Stiffening element 100B may comprise one or more integral currentflowpaths. Aluminum layer 102B (e.g., one or more of aluminum sublayers102B1, 102B2, 102B3, or 102B4) may be configured to form at least partof the one or more integral current flowpaths.

FIG. 1C shows stiffening element 100C, according to some examples of thedisclosed stiffening elements. As shown in FIG. 1C, stiffening element100C comprises: aluminum layer 102C; glass composite layer 104C adjacentto aluminum layer 102C; and carbon composite layer 106C adjacent toglass composite layer 104C, and opposite to aluminum layer 102C.

Glass composite layer 104C may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. Carbon compositelayer 106C may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies.

Glass composite layer 104C may comprise a plurality of glass compositesublayers 104C1, 104C2, 104C3, 104C4 (e.g., 104C1, 104C2, . . . ,104Cn). The number (n) of glass composite sublayers may be, for example,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers.

Aluminum layer 102C, glass composite layer 104C, and carbon compositelayer 106C may have the same or different thicknesses. Similarly,plurality of glass composite sublayers 104C1, 104C2, 104C3, 104C4 mayhave the same or different thicknesses.

Aluminum layer 102C may be configured to form the outer surface ofstiffening element 100C or an outer surface of stiffening element 100C.

Stiffening element 100C may comprise one or more integral currentflowpaths. Aluminum layer 102C may be configured to form at least partof the one or more integral current flowpaths.

FIG. 1D shows stiffening element 100D, according to some examples of thedisclosed stiffening elements. As shown in FIG. 1D, stiffening element100D comprises: aluminum layer 102D; glass composite layer 104D adjacentto aluminum layer 102D; and carbon composite layer 106D adjacent toglass composite layer 104D, and opposite to aluminum layer 102D.

Glass composite layer 104D may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. Carbon compositelayer 106D may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies.

Carbon composite layer 106D may comprise a plurality of carbon compositesublayers 106D1, 106D2, 106D3, 106D4 (e.g., 106D1, 106D2, . . . ,106Dn). The number (n) of carbon composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or more sublayers(e.g., 8, 16, 24, 32, 40, or 64 sublayers).

Aluminum layer 102D, glass composite layer 104D, and carbon compositelayer 106D may have the same or different thicknesses. Similarly,plurality of carbon composite sublayers 106D1, 106D2, 106D3, 106D4 mayhave the same or different thicknesses.

Aluminum layer 102D may be configured to form the outer surface ofstiffening element 100D or an outer surface of stiffening element 100D.

Stiffening element 100D may comprise one or more integral currentflowpaths. Aluminum layer 102D may be configured to form at least partof the one or more integral current flowpaths.

FIG. 1E shows stiffening element 100E, according to some examples of thedisclosed stiffening elements. As shown in FIG. 1E, stiffening element100E comprises: aluminum layer 102E; glass composite layer 104E adjacentto aluminum layer 102E; and carbon composite layer 106E adjacent toglass composite layer 104E, and opposite to aluminum layer 102E.

Glass composite layer 104E may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. Carbon compositelayer 106E may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies.

Aluminum layer 102E may comprise a plurality of aluminum sublayers102E1, 102E2 (e.g., 102E1, 102E2, . . . , 102En). The number (n) ofaluminum sublayers may be, for example, 2, 3, 4, 5, 6, or moresublayers. Glass composite layer 104E may comprise a plurality of glasscomposite sublayers 104E1, 104E2, 104E3, 104E4 (e.g., 104E1, 104E2, . .. , 104Eo). The number (o) of glass composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers. Carboncomposite layer 106E may comprise a plurality of carbon compositesublayers 106E1, 106E2, 106E3 (e.g., 106E1, 106E2, . . . , 106Ep). Thenumber (p) of carbon composite sublayers may be, for example, 2, 3, 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,24, 25, 26, 27, 28, 29, 30, 31, 32, or more sublayers (e.g., 8, 16, 24,32, 40, or 64 sublayers).

The number of aluminum sublayers may be the same as or differ from thenumber of glass composite sublayers. The number of aluminum sublayersmay be the same as or differ from the number of carbon compositesublayers. The number of glass composite sublayers may be the same as ordiffer from the number of carbon composite sublayers.

Aluminum layer 102E, glass composite layer 104E, and carbon compositelayer 106E may have the same or different thicknesses. Plurality ofplurality of aluminum sublayers 102E1, 102E2 may have the same ordifferent thicknesses. Plurality of glass composite sublayers 104E1,104E2, 104E3, 104E4 may have the same or different thicknesses.Plurality of carbon composite sublayers 106E1, 106E2, 106E3 may have thesame or different thicknesses.

Aluminum layer 102E (e.g., aluminum sublayer 102E1) may be configured toform the outer surface of stiffening element 100E or an outer surface ofstiffening element 100E.

Stiffening element 100E may comprise one or more integral currentflowpaths. Aluminum layer 102E (e.g., one or both of aluminum sublayers102E1 or 102E2) may be configured to form at least part of the one ormore integral current flowpaths.

The glass composite layer (e.g., glass composite layer 104A, 104B, 104C,104D, 104E) may comprise first thermoplastic resin. The firstthermoplastic resin may comprise PEEK. The first thermoplastic resin maycomprise PEKK. the first thermoplastic resin may comprise one or more ofPAEK, PEI, or PPS. The first thermoplastic resin may comprise, forexample, one or more of PAEK, PEEK, PEEKK, PEI, PEK, PEKEKK, PEKK, orPPS. The carbon composite layer (e.g., carbon composite layer 106A,106B, 106C, 106D, 106E) may comprise second thermoplastic resin. Thesecond thermoplastic resin may comprise PEEK. The second thermoplasticresin may comprise PEKK. The second thermoplastic resin may comprise oneor more of PAEK, PEI, or PPS. The second thermoplastic resin maycomprise, for example, one or more of PAEK, PEEK, PEEKK, PEI, PEK,PEKEKK, PEKK, or PPS. The first thermoplastic resin may be the same asthe second thermoplastic resin. The first thermoplastic resin may differfrom the second thermoplastic resin.

FIG. 2A shows stiffening element 200A, according to some examples of thedisclosed stiffening elements. As shown in FIG. 2A, stiffening element200A comprises: first aluminum layer 202A; first glass composite layer204A adjacent to first aluminum layer 202A; first carbon composite layer206A adjacent to first glass composite layer 204A, and opposite to firstaluminum layer 202A; and second glass composite layer 208A adjacent tofirst carbon composite layer 206A, and opposite to first glass compositelayer 204A.

First aluminum layer 202A may be configured to form the outer surface ofstiffening element 200A or an outer surface of stiffening element 200A.

Stiffening element 200A may comprise one or more integral currentflowpaths. Aluminum layer 202A may be configured to form at least partof the one or more integral current flowpaths.

First aluminum layer 202A may comprise a plurality of first aluminumsublayers (not shown). The number of first aluminum sublayers may be,for example, 2, 3, 4, 5, 6, or more sublayers. First glass compositelayer 204A may comprise a plurality of first glass composite sublayers(not shown). The number of first glass composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers. Each orall of the first glass composite sublayers may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. First carboncomposite layer 206A may comprise a plurality of first carbon compositesublayers (not shown). The number of first carbon composite sublayersmay be, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or moresublayers (e.g., 8, 16, 24, 32, 40, or 64 sublayers). Each or all of thefirst carbon composite sublayers may comprise one or morecarbon-fiber-reinforced thermoplastic prepreg plies.

Second glass composite layer 208A may comprise a plurality of secondglass composite sublayers (not shown). The number of second glasscomposite sublayers may be, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, or more sublayers. Each or all of the sublayers may comprise one ormore glass-fiber-reinforced thermoplastic prepreg plies.

First glass composite layer 204A may comprise one or more firstglass-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise first thermoplastic resin. First carbon composite layer 206Amay comprise one or more first carbon-fiber-reinforced thermoplasticprepreg plies which, in turn, may comprise second thermoplastic resin.Second glass composite layer 208A may comprise one or more secondglass-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise third thermoplastic resin. The first thermoplastic resin may bethe same as or differ from the second thermoplastic resin. The firstthermoplastic resin may be the same as or differ from the thirdthermoplastic resin. The second thermoplastic resin may be the same asor differ from the third thermoplastic resin.

The first thermoplastic resin may comprise PEEK. The first thermoplasticresin may comprise PEKK. The first thermoplastic resin may comprise oneor more of PAEK, PEI, or PPS. The first thermoplastic resin maycomprise, for example, one or more of PAEK, PEEK, PEEKK, PEI, PEK,PEKEKK, PEKK, or PPS. The second thermoplastic resin may comprise PEEK.The second thermoplastic resin may comprise PEKK. The secondthermoplastic resin may comprise one or more of PAEK, PEI, or PPS. Thesecond thermoplastic resin may comprise, for example, one or more ofPAEK, PEEK, PEEKK, PEI, PEK, PEKEKK, PEKK, or PPS. The thirdthermoplastic resin may comprise PEEK. The third thermoplastic resin maycomprise PEKK. The third thermoplastic resin may comprise one or more ofPAEK, PEI, or PPS. The third thermoplastic resin may comprise, forexample, one or more of PAEK, PEEK, PEEKK, PEI, PEK, PEKEKK, PEKK, orPPS.

FIG. 2B shows stiffening element 200B, according to some examples of thedisclosed stiffening elements. As shown in FIG. 2B, stiffening element200B comprises: first aluminum layer 202B; first glass composite layer204B adjacent to first aluminum layer 202B; first carbon composite layer206B adjacent to first glass composite layer 204B, and opposite to firstaluminum layer 202B; second glass composite layer 208B adjacent to firstcarbon composite layer 206B, and opposite to first glass composite layer204B; and second aluminum layer 210B adjacent to second glass compositelayer 208B, and opposite to first carbon composite layer 206B.

First glass composite layer 204B may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. First carboncomposite layer 206B may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies. Second glass composite layer 208B maycomprise one or more glass-fiber-reinforced thermoplastic prepreg plies.

First aluminum layer 202B may be configured to form the outer surface ofstiffening element 200B or an outer surface of stiffening element 200B.Second aluminum layer 210B may be configured to form the outer surfaceof stiffening element 200B or an outer surface of stiffening element200B. First aluminum layer 202B may be configured to form a first outersurface of stiffening element 200B, while second aluminum layer 210B maybe configured to form a second outer surface of stiffening element 200B.The first outer surface of stiffening element 200B may be substantiallyparallel to the second outer surface of stiffening element 200B.

Stiffening element 200B may comprise one or more integral currentflowpaths. First aluminum layer 202B may be configured to form at leastpart of the one or more integral current flowpaths. Second aluminumlayer 210B may be configured to form at least part of the one or moreintegral current flowpaths.

First aluminum layer 202B may be configured to form a first flowpath ofthe one or more integral current flowpaths. Second aluminum layer 210Bmay be configured to form a second flowpath of the one or more integralcurrent flowpaths. The first flowpath may differ from the secondflowpath (e.g., independent from each other). The first and secondflowpaths may be part of a same integral current flowpath.

Current flow in the first flowpath may be substantially parallel tocurrent flow in the second flowpath. Current flow in the first flowpathmay be substantially in a same direction as current flow in the secondflowpath. Current flow in the first flowpath may be substantially in anopposite direction from current flow in the second flowpath. Currentflow in the first flowpath may be substantially parallel to and in asame direction as current flow in the second flowpath. Current flow inthe first flowpath may be substantially parallel to but in an oppositedirection from current flow in the second flowpath.

First aluminum layer 202B may comprise a plurality of first aluminumsublayers (not shown). The number of first aluminum sublayers may be,for example, 2, 3, 4, 5, 6, or more sublayers. First glass compositelayer 204B may comprise a plurality of first glass composite sublayers(not shown). The number of first glass composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers. Each orall of the first glass composite sublayers may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. First carboncomposite layer 206B may comprise a plurality of first carbon compositesublayers (not shown). The number of first carbon composite sublayersmay be, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or moresublayers (e.g., 8, 16, 24, 32, 40, or 64 sublayers). Each or all of thefirst carbon composite sublayers may comprise one or morecarbon-fiber-reinforced thermoplastic prepreg plies.

Second aluminum layer 210B may comprise a plurality of second aluminumsublayers (not shown). The number of second aluminum sublayers may be,for example, 2, 3, 4, 5, 6, or more sublayers.

First aluminum layer 202B and second aluminum layer 210B may comprisethe same aluminum alloy. First aluminum layer 202B and second aluminumlayer 210B may comprise different aluminum alloys.

FIG. 2C shows stiffening element 200C, according to some examples of thedisclosed stiffening elements. As shown in FIG. 2C, stiffening element200C comprises: first aluminum layer 202C; first glass composite layer204C adjacent to first aluminum layer 202C; first carbon composite layer206C adjacent to first glass composite layer 204C, and opposite to firstaluminum layer 202C; second glass composite layer 208C adjacent to firstcarbon composite layer 206C, and opposite to first glass composite layer204C; and second carbon composite layer 212C adjacent to second glasscomposite layer 208C, and opposite to first carbon composite layer 206C.

First aluminum layer 202C may comprise a plurality of first aluminumsublayers (not shown). The number of first aluminum sublayers may be,for example, 2, 3, 4, 5, 6, or more sublayers. First glass compositelayer 204C may comprise a plurality of first glass composite sublayers(not shown). The number of first glass composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers. Each orall of the first glass composite sublayers may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. First carboncomposite layer 206C may comprise a plurality of first carbon compositesublayers (not shown). The number of first carbon composite sublayersmay be, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or moresublayers (e.g., 8, 16, 24, 32, 40, or 64 sublayers). Each or all of thefirst carbon composite sublayers may comprise one or morecarbon-fiber-reinforced thermoplastic prepreg plies.

Second carbon composite layer 212C may comprise a plurality of secondcarbon composite sublayers (not shown). The number of second carboncomposite sublayers may be, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,30, 31, 32, or more sublayers (e.g., 8, 16, 24, 32, 40, or 64sublayers). Each or all of the sublayers may comprise one or morecarbon-fiber-reinforced thermoplastic prepreg plies.

First glass composite layer 204C may comprise one or more firstglass-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise first thermoplastic resin. First carbon composite layer 206Cmay comprise one or more first carbon-fiber-reinforced thermoplasticprepreg plies which, in turn, may comprise second thermoplastic resin.Second glass composite layer 208C may comprise one or more secondglass-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise third thermoplastic resin. Second carbon composite layer 212Cmay comprise one or more second carbon-fiber-reinforced thermoplasticprepreg plies which, in turn, may comprise fourth thermoplastic resin.The first thermoplastic resin may be the same as or differ from thesecond thermoplastic resin. The first thermoplastic resin may be thesame as or differ from the third thermoplastic resin. The firstthermoplastic resin may be the same as or differ from the fourththermoplastic resin. The second thermoplastic resin may be the same asor differ from the third thermoplastic resin. The second thermoplasticresin may be the same as or differ from the fourth thermoplastic resin.The third thermoplastic resin may be the same as or differ from thefourth thermoplastic resin.

The first thermoplastic resin may comprise PEEK. The first thermoplasticresin may comprise PEKK. The first thermoplastic resin may comprise oneor more of PAEK, PEI, or PPS. The first thermoplastic resin maycomprise, for example, one or more of PAEK, PEEK, PEEKK, PEI, PEK,PEKEKK, PEKK, or PPS. The second thermoplastic resin may comprise PEEK.The second thermoplastic resin may comprise PEKK. The secondthermoplastic resin may comprise one or more of PAEK, PEI, or PPS. Thesecond thermoplastic resin may comprise, for example, one or more ofPAEK, PEEK, PEEKK, PEI, PEK, PEKEKK, PEKK, or PPS. The thirdthermoplastic resin may comprise PEEK. The third thermoplastic resin maycomprise PEKK. The third thermoplastic resin may comprise one or more ofPAEK, PEI, or PPS. The third thermoplastic resin may comprise, forexample, one or more of PAEK, PEEK, PEEKK, PEI, PEK, PEKEKK, PEKK, orPPS. The fourth thermoplastic resin may comprise PEEK. The fourththermoplastic resin may comprise PEKK. The fourth thermoplastic resinmay comprise one or more of PAEK, PEI, or PPS. The fourth thermoplasticresin may comprise, for example, one or more of PAEK, PEEK, PEEKK, PEI,PEK, PEKEKK, PEKK, or PPS.

FIG. 2D shows stiffening element 200D, according to some examples of thedisclosed stiffening elements. As shown in FIG. 2D, stiffening element200D may comprise: first aluminum layer 202D; first glass compositelayer 204D adjacent to first aluminum layer 202D; first carbon compositelayer 206D adjacent to first glass composite layer 204D, and opposite tofirst aluminum layer 202D; second glass composite layer 208D adjacent tofirst carbon composite layer 206D, and opposite to first glass compositelayer 204D; second aluminum layer 210D adjacent to second glasscomposite layer 208D, and opposite to first carbon composite layer 206D;third glass composite layer 214D adjacent to second aluminum layer 210D,and opposite to second glass composite layer 208D; second carboncomposite layer 216D adjacent to third glass composite layer 214D, andopposite to second aluminum layer 210D; fourth glass composite layer218D adjacent to second carbon composite layer 216D, and opposite tothird glass composite layer 214D; and third aluminum layer 220D adjacentto fourth glass composite layer 218D, and opposite to second carboncomposite layer 216D.

Each of the aluminum layers may comprise a plurality of aluminumsublayers (not shown). The number of aluminum sublayers may be, forexample, 2, 3, 4, 5, 6, or more sublayers. Each of the glass compositelayers may comprise a plurality of glass composite sublayers (notshown). The number of glass composite sublayers may be, for example, 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers. Each of the carboncomposite layers may comprise a plurality of carbon composite sublayers(not shown). The number of carbon composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or more sublayers(e.g., 8, 16, 24, 32, 40, or 64 sublayers).

First aluminum layer 202D may be configured to form the outer surface ofstiffening element 200D or an outer surface of stiffening element 200D.Third aluminum layer 220D may be configured to form the outer surface ofstiffening element 200D or an outer surface of stiffening element 200D.First aluminum layer 202D may be configured to form a first outersurface of stiffening element 200D, while third aluminum layer 220D maybe configured to form a second outer surface of stiffening element 200D.The first outer surface of stiffening element 200D may be substantiallyparallel to the second outer surface of stiffening element 200D.

Stiffening element 200D may comprise one or more integral currentflowpaths. First aluminum layer 202D may be configured to form at leastpart of the one or more integral current flowpaths. Second aluminumlayer 210D may be configured to form at least part of the one or moreintegral current flowpaths. Third aluminum layer 220D may be configuredto form at least part of the one or more integral current flowpaths.

First aluminum layer 202D may be configured to form a first flowpath ofthe one or more integral current flowpaths. Third aluminum layer 220Dmay be configured to form a second flowpath of the one or more integralcurrent flowpaths. The first flowpath may differ from the secondflowpath (e.g., independent from each other). The first and secondflowpaths may be part of a same integral current flowpath.

Current flow in the first flowpath may be substantially parallel tocurrent flow in the second flowpath. Current flow in the first flowpathmay be substantially in a same direction as current flow in the secondflowpath. Current flow in the first flowpath may be substantially in anopposite direction from current flow in the second flowpath. Currentflow in the first flowpath may be substantially parallel to and in asame direction as current flow in the second flowpath. Current flow inthe first flowpath may be substantially parallel to but in an oppositedirection from current flow in the second flowpath.

First aluminum layer 202D may be configured to form a first flowpath ofthe one or more integral current flowpaths. Second aluminum layer 210Dmay be configured to form a second flowpath of the one or more integralcurrent flowpaths. Third aluminum layer 220D may be configured to form athird flowpath of the one or more integral current flowpaths. The firstflowpath may differ from the second flowpath (e.g., independent fromeach other), the first flowpath may differ from the third flowpath(e.g., independent from each other), and second flowpath may differ fromthe third flowpath (e.g., independent from each other).

Current flow in any of the first, second, or third flowpaths may besubstantially parallel to current flow in either of the other twoflowpaths. Current flow in any of the first, second, or third flowpathsmay be substantially in a same direction as current flow in either ofthe other two flowpaths. Current flow in any of the first, second, orthird flowpaths may be substantially in an opposite direction fromcurrent flow in either of the other two flowpaths. Current flow in anyof the first, second, or third flowpaths may be substantially parallelto and in a same direction as current flow in in either of the other twoflowpaths. Current flow in any of the first, second, or third flowpathsmay be substantially parallel to but in an opposite direction fromcurrent flow in either of the other two flowpaths.

First aluminum layer 202D and second aluminum layer 210D may comprisethe same aluminum alloy. First aluminum layer 202D and third aluminumlayer 220D may comprise the same aluminum alloy. Second aluminum layer210D and third aluminum layer 220D may comprise the same aluminum alloy.First aluminum layer 202D, second aluminum layer 210D, and thirdaluminum layer 220D may comprise the same aluminum alloy.

First aluminum layer 202D and second aluminum layer 210D may comprisedifferent aluminum alloys. First aluminum layer 202D and third aluminumlayer 220D may comprise different aluminum alloys. Second aluminum layer210D and third aluminum layer 220D may comprise different aluminumalloys. First aluminum layer 202D, second aluminum layer 210D, and thirdaluminum layer 220D each may comprise different aluminum alloys.

Each or all of the glass composite layers may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise first thermoplastic resin. The first thermoplastic resin maycomprise PEEK. The first thermoplastic resin may comprise PEKK. Thefirst thermoplastic resin may comprise one or more of PAEK, PEI, or PPS.The first thermoplastic resin may comprise one or more of PAEK, PEEK,PEEKK, PEI, PEK, PEKEKK, PEKK, or PPS. The first thermoplastic resin maybe the same as or differ from the thermoplastic resin in any other glassor carbon composite layer(s).

Each or all of the carbon composite layers may comprise one or morecarbon-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise second thermoplastic resin. In each or all of the carboncomposite layers, the second thermoplastic resin may comprise PEEK. Ineach or all of the carbon composite layers, the second thermoplasticresin may comprise PEKK. In each or all of the carbon composite layers,the second thermoplastic resin may comprise one or more of PAEK, PEI, orPPS. In each or all of the carbon composite layers, the secondthermoplastic resin may comprise one or more of PAEK, PEEK, PEEKK, PEI,PEK, PEKEKK, PEKK, or PPS. In each of the carbon composite layers, thesecond thermoplastic resin may be the same as or differ from thethermoplastic resin in any other carbon or glass composite layer(s).

FIG. 2E shows stiffening element 200E, according to some examples of thedisclosed stiffening elements. As shown in FIG. 2E, stiffening element200E may comprise: first aluminum layer 202E; first glass compositelayer 204E adjacent to first aluminum layer 202E; first carbon compositelayer 206E adjacent to first glass composite layer 204E, and opposite tofirst aluminum layer 202E; second glass composite layer 208E adjacent tofirst carbon composite layer 206E, and opposite to first glass compositelayer 204E; second carbon composite layer 212E adjacent to second glasscomposite layer 208E, and opposite to first carbon composite layer 206E;third glass composite layer 222E adjacent to second carbon compositelayer 212E, and opposite to second glass composite layer 208E; thirdcarbon composite layer 224E adjacent to third glass composite layer222E, and opposite to second carbon composite layer 212E; and/or fourthglass composite layer 226E adjacent to third carbon composite layer224E, and opposite to third glass composite layer 222E.

First aluminum layer 202E may comprise a plurality of first aluminumsublayers (not shown). The number of first aluminum sublayers may be,for example, 2, 3, 4, 5, 6, or more sublayers. Each of the glasscomposite layers may comprise a plurality of glass composite sublayers(not shown). The number of glass composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers. Each ofthe carbon composite layers may comprise a plurality of carbon compositesublayers (not shown). The number of carbon composite sublayers may be,for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or moresublayers (e.g., 8, 16, 24, 32, 40, or 64 sublayers).

Each or all of the glass composite layers may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise first thermoplastic resin. In each or all of the glasscomposite layers, the first thermoplastic resin may comprise PEEK. Ineach or all of the glass composite layers, the first thermoplastic resinmay comprise PEKK. In each or all of the glass composite layers, thefirst thermoplastic resin may comprise one or more of PAEK, PEI, or PPS.In each or all of the glass composite layers, the first thermoplasticresin may comprise one or more of PAEK, PEEK, PEEKK, PEI, PEK, PEKEKK,PEKK, or PPS. In each of the glass composite layers, the firstthermoplastic resin may be the same as or differ from the thermoplasticresin in any other glass or carbon composite layer(s).

Each or all of the carbon composite layers may comprise one or morecarbon-fiber-reinforced thermoplastic prepreg plies which, in turn, maycomprise second thermoplastic resin. In each or all of the carboncomposite layers, the second thermoplastic resin may comprise PEEK. Ineach or all of the carbon composite layers, the second thermoplasticresin may comprise PEKK. In each or all of the carbon composite layers,the second thermoplastic resin may comprise one or more of PAEK, PEI, orPPS. In each or all of the carbon composite layers, the secondthermoplastic resin may comprise one or more of PAEK, PEEK, PEEKK, PEI,PEK, PEKEKK, PEKK, or PPS. In each of the carbon composite layers, thesecond thermoplastic resin may be the same as or differ from thethermoplastic resin in any other carbon or glass composite layer(s).

FIG. 3A shows laying up a first layer on a mold tool (e.g., a mandrel),according to some examples of the disclosed stiffening elements. Asshown in FIG. 3A, first layer 302 may be laid up on mold tool 300. Moldtool 300 may be of substantially any shape. First layer 302 maycomprise, for example, an aluminum layer, a glass composite layer, or acarbon composite layer. To facilitate subsequent removal of a stiffeningelement from mold tool 300, a parting film or similar may be usedbetween mold tool 300 and first layer 302. First layer 302 may comprisetwo or more sublayers (not shown).

FIG. 3B shows laying up a second layer on the first layer of FIG. 3A,according to some examples of the disclosed stiffening elements. Asshown in FIG. 3B, second layer 304 may be laid up on first layer 302 toform a first portion of stack 318 (see FIG. 3D). Second layer 304 maycomprise, for example, an aluminum layer, a glass composite layer, or acarbon composite layer, provided that an aluminum layer does notdirectly contact a carbon composite layer (due to galvanic corrosionconcerns). Second layer 304 may comprise two or more sublayers (notshown). The shape of second layer 304 may be the same as or differ fromthe shape of first layer 302. The thickness of second layer 304 may bethe same as or differ from the thickness of first layer 302.

Stack 318 may include an additional layer (not shown) between firstlayer 302 and second layer 304. The additional layer may improve thebonding of first layer 302 and second layer 304. The additional layermay be, for example, an adhesive layer. In such cases, second layer 304may be adjacent to first layer 302, but may not directly contact firstlayer 302. Care should be taken during selection of material(s) for suchan additional layer because, for example, some material(s) compriseelements or compounds that may interact with first layer 302 and/orsecond layer 304 via one or more interaction mechanisms.

FIG. 3C shows laying up a third layer on the second layer of FIG. 3B,according to some examples of the disclosed stiffening elements. Asshown in FIG. 3C, third layer 306 may be laid up on second layer 304 toform a second portion of stack 318. Third layer 306 may comprise, forexample, an aluminum layer, a glass composite layer, or a carboncomposite layer, as long as an aluminum layer does not directly contacta carbon composite layer (due to galvanic corrosion concerns). Thirdlayer 306 may comprise two or more sublayers (not shown). The shape ofthird layer 306 may be the same as or differ from the shape of firstlayer 302. The thickness of third layer 306 may be the same as or differfrom the thickness of first layer 302. The shape of third layer 306 maybe the same as or differ from the shape of second layer 304. Thethickness of third layer 306 may be the same as or differ from thethickness of second layer 304.

Stack 318 may include an additional layer (not shown) between secondlayer 304 and third layer 306. The additional layer may improve thebonding of second layer 304 and third layer 306. The additional layermay be, for example, an adhesive layer. In such cases, third layer 306may be adjacent to second layer 304, but may not directly contact secondlayer 304. Care should be taken during selection of material(s) for suchan additional layer because, for example, some material(s) compriseelements or compounds that may interact with second layer 304 and/orthird layer 306 via one or more interaction mechanisms.

In addition to addressing galvanic corrosion concerns, a glass compositelayer also may function as a compliant layer, reducing strain withinand/or stress between layers adjacent to the glass composite layer(e.g., an aluminum layer on one side and a carbon fiber layer on theother). Such a compliant layer may at least partially decouple effects(e.g., thermal contraction, thermal expansion, strains, or stresses)associated with the layers adjacent to the glass composite layer, and/orreduce strain within and/or stress between layers through effectsassociated with orientation of glass fibers in the glass compositelayer(s).

The addition of layers may continue until a desired stacking of layersis achieved. The laying up may be done manually and/or automatically.

Surfaces of aluminum layers may undergo surface preparation, such asalkaline degreasing, chromic acid anodizing or other anodizingprocessing, priming (e.g., with BR 127 corrosion-inhibiting primer),sol-gel, and/or pickling in chromic-sulfuric acid. The surfaces also maybe roughened, for example, by abrasion.

Stack 318 may be bagged (e.g., vacuum bag) on the mold tool (e.g., amandrel).

The stacking and consolidating may use stationary compression molding(e.g., autoclave) or continuous compression molding (“CCM”) (e.g.,out-of-autoclave or vacuum-bag-only). These techniques may allowstiffening elements to be formed in a single manufacturing operation,such as a single-step CCM thermoplastic consolidation process.

FIG. 3D shows stack 318 comprising a first carbon composite layer, afirst glass composite layer, a second carbon composite layer, a secondglass composite layer, and/or an aluminum layer, according to someexamples of the disclosed stiffening elements. As shown in FIG. 3D,first carbon composite layer 308 may be laid up on mold tool 300, firstglass composite layer 310 may be laid up on first carbon composite layer308, second carbon composite layer 312 may be laid up on first glasscomposite layer 310, second glass composite layer 314 may be laid up onsecond carbon composite layer 312, and/or aluminum layer 316 may be laidup on second glass composite layer 314 to achieve stack 318.

First glass composite layer 310 may be adjacent to first carboncomposite layer 308, second carbon composite layer 312 may be adjacentto first glass composite layer 310, second glass composite layer 314 maybe adjacent to second carbon composite layer 312, and/or aluminum layer316 may be adjacent to second glass composite layer 314.

One or more of first carbon composite layer 308, first glass compositelayer 310, second carbon composite layer 312, second glass compositelayer 314, or aluminum layer 316 may comprise two or more sublayers (notshown).

Stack 318 may include additional layer(s) (not shown) between firstcarbon composite layer 308 and first glass composite layer 310; betweenfirst glass composite layer 310 and second carbon composite layer 312;between second carbon composite layer 312 and second glass compositelayer 314; and/or between second glass composite layer 314 and aluminumlayer 316. The additional layer(s) may improve the bonding of associatedadjacent layers. The additional layer(s) may be, for example, adhesivelayer(s). In such cases, layers may be adjacent to each other, but maynot directly contact each other. Care should be taken during selectionof material(s) for such additional layer(s) because, for example, somematerial(s) comprise elements or compounds that may interact with otherlayers via one or more interaction mechanisms.

One or both of first glass composite layer 310 or second glass compositelayer 314 may comprise one or more thermoplastic prepreg plies. The oneor more thermoplastic prepreg plies may be consolidated at a temperaturesufficient to soften aluminum layer 316. The consolidating of the one ormore thermoplastic prepreg plies may be conducted, for example, in anautoclave.

The consolidating of the one or more thermoplastic prepreg plies of oneor both of first glass composite layer 310 or second glass compositelayer 314 comprises raising a temperature of the one or morethermoplastic prepreg plies to ≥600° F., ≥650° F., ≥675° F., or ≥700° F.(e.g., 707° F.±9° F. or 710° F.±10° F.); raising a pressure of the oneor more thermoplastic prepreg plies to ≥50 psig, ≥100 psig, ≥150 psig,≥200 psig, ≥250 psig, or ≤300 psig (e.g., 100 psig±5 psig or 290 psigminimum); and/or holding the temperature and pressure for 5 minutes, 10minutes, ≥15 minutes, or ≥20 minutes (e.g., 20 minutes+15 minutes/−5minutes). For example, the temperature of the one or more thermoplasticprepreg plies may be raised to 710° F.±10° F., the pressure adjusted to100 psig±5 psig, and then the temperature and pressure may be heldwithin those ranges for 20 minutes+15 minutes/−5 minutes. For example,the temperature of the one or more thermoplastic prepreg plies may beraised to 707° F.±9° F., the pressure adjusted to 290 psig minimum, andthen the temperature and pressure may be held within those temperatureand pressure ranges for 6 minutes minimum.

The consolidating of the one or more thermoplastic prepreg plies of oneor both of first glass composite layer 310 or second glass compositelayer 314 comprises raising a temperature of stack 318 to ≥600° F.,≥650° F., ≥675° F., or ≥700° F. (e.g., 707° F.±9° F. or 710° F.±10° F.);raising a pressure of stack 318 to ≥50 psig, ≥100 psig, ≥150 psig, ≥200psig, ≥250 psig, or ≤300 psig (e.g., 100 psig±5 psig or 290 psigminimum); and/or holding the temperature and pressure for ≥5 minutes,≥10 minutes, ≥15 minutes, or ≥20 minutes (e.g., 20 minutes+15 minutes/−5minutes). For example, the temperature of stack 318 may be raised to710° F.±10° F., the pressure adjusted to 100 psig±5 psig, and then thetemperature and pressure may be held within those temperature andpressure ranges for 20 minutes+15 minutes/−5 minutes. For example, thetemperature of stack 318 may be raised to 707° F.±9° F., the pressureadjusted to 290 psig minimum, and then the temperature and pressure maybe held within those temperature and pressure ranges for 6 minutesminimum.

One or both of first carbon composite layer 308 or second carboncomposite layer 312 may comprise one or more thermoplastic prepregplies. The one or more thermoplastic prepreg plies may be consolidatedat a temperature sufficient to soften aluminum layer 316. Theconsolidating of the one or more thermoplastic prepreg plies may beconducted, for example, in an autoclave.

The consolidating of the one or more thermoplastic prepreg plies of oneor both of first carbon composite layer 308 or second carbon compositelayer 312 comprises raising a temperature of the one or morethermoplastic prepreg plies to ≥600° F., ≥650° F., ≥675° F., or ≥700° F.(e.g., 707° F.±9° F. or 710° F.±10° F.); raising a pressure of the oneor more thermoplastic prepreg plies to ≥50 pounds per square inch gage(“psig”), ≥100 psig, ≥150 psig, ≥200 psig, ≥250 psig, or ≤300 psig(e.g., 100 psig±5 psig or 290 psig minimum); and/or holding thetemperature and pressure for ≥5 minutes, ≥10 minutes, ≥15 minutes, or≥20 minutes (e.g., 20 minutes+15 minutes/−5 minutes). For example, thetemperature of the one or more thermoplastic prepreg plies may be raisedto 710° F.±10° F., the pressure adjusted to 100 psig±5 psig, and thenthe temperature and pressure may be held within those ranges for 20minutes+15 minutes/−5 minutes. For example, the temperature of the oneor more thermoplastic prepreg plies may be raised to 707° F.±9° F., thepressure adjusted to 290 psig minimum, and then the temperature andpressure may be held within those temperature and pressure ranges for 6minutes minimum.

The consolidating of the one or more thermoplastic prepreg plies of oneor both of first carbon composite layer 308 or second carbon compositelayer 312 comprises raising a temperature of stack 318 to ≥600° F.,≥650° F., ≥675° F., or ≥700° F. (e.g., 707° F.±9° F. or 710° F.±10° F.);raising a pressure of stack 318 to ≥50 psig, ≥100 psig, ≥150 psig, ≥200psig, ≥250 psig, or ≤300 psig (e.g., 100 psig±5 psig or 290 psigminimum); and/or holding the temperature and pressure for ≥5 minutes,≥10 minutes, ≥15 minutes, or ≥20 minutes (e.g., 20 minutes+15 minutes/−5minutes). For example, the temperature of stack 318 may be raised to710° F.±10° F., the pressure adjusted to 100 psig±5 psig, and then thetemperature and pressure may be held within those temperature andpressure ranges for 20 minutes+15 minutes/−5 minutes. For example, thetemperature of stack 318 may be raised to 707° F.±9° F., the pressureadjusted to 290 psig minimum, and then the temperature and pressure maybe held within those temperature and pressure ranges for 6 minutesminimum.

Such thermoplastic prepreg plies may be produced in advance and stored,for example, on rolls with backing paper. Prepreg tapes forthermoplastic prepreg plies, if unidirectional, may be produced, forexample, by extrusion or pultrusion.

The thermoplastic prepreg plies may be dried (e.g., in an oven) prior toconsolidation. For example, the thermoplastic prepreg plies may be driedat 250° F. for a minimum of 10 hours prior to consolidation.

One or both of first glass composite layer 310 or second glass compositelayer 314 may comprise first thermoplastic resin. The temperature andpressure of stack 318 may be adjusted so as to consolidate stack 318when forming a stiffening element. The adjusting of the temperature andpressure of stack 318 comprises raising a temperature of the firstthermoplastic resin to ≥600° F., ≥650° F., ≥675° F., or ≥700° F. (e.g.,707° F.±9° F. or 710° F.±10° F.); raising a pressure of the firstthermoplastic resin to ≥50 psig, ≥100 psig, ≥150 psig, ≥200 psig, ≥250psig, or ≤300 psig (e.g., 100 psig±5 psig or 290 psig minimum); and/orholding the temperature and pressure for ≥5 minutes, ≥10 minutes, ≥15minutes, or ≥20 minutes (e.g., 20 minutes+15 minutes/−5 minutes). Forexample, the temperature of the first thermoplastic resin may be raisedto 710° F.±10° F., the pressure adjusted to 100 psig±5 psig, and thenthe temperature and pressure may be held within those ranges for 20minutes+15 minutes/−5 minutes. For example, the temperature of the firstthermoplastic resin may be raised to 707° F.±9° F., the pressureadjusted to 290 psig minimum, and then the temperature and pressure maybe held within those temperature and pressure ranges for 6 minutesminimum.

One or both of first carbon composite layer 308 or second carboncomposite layer 312 may comprise second thermoplastic resin. Thetemperature and pressure of the second thermoplastic resin may beadjusted so as to consolidate stack 318 when forming a stiffeningelement. The adjusting of the temperature and pressure of stack 318comprises raising a temperature of the second thermoplastic resin to≥600° F., ≥650° F., ≥675° F., or ≥700° F. (e.g., 707° F.±9° F. or 710°F.±10° F.); raising a pressure of the second thermoplastic resin to ≥50psig, ≥100 psig, ≥150 psig, ≥200 psig, ≥250 psig, or ≤300 psig (e.g.,100 psig±5 psig or 290 psig minimum); and/or holding the temperature andpressure for ≥5 minutes, ≥10 minutes, ≥15 minutes, or ≥20 minutes (e.g.,20 minutes+15 minutes/−5 minutes). For example, the temperature of thesecond thermoplastic resin may be raised to 710° F.±10° F., the pressureadjusted to 100 psig±5 psig, and then the temperature and pressure maybe held within those ranges for 20 minutes+15 minutes/−5 minutes. Forexample, the temperature of the second thermoplastic resin may be raisedto 707° F.±9° F., the pressure adjusted to 290 psig minimum, and thenthe temperature and pressure may be held within those temperature andpressure ranges for 6 minutes minimum.

A temperature versus time profile for consolidating the one or morethermoplastic prepreg plies at a temperature sufficient to softenaluminum layer 316, or for adjusting the temperature and pressure ofstack 318 so as to consolidate stack 318 may comprise three phases: aheat-up phase (generally positive slope), a hold phase (generally zeroslope), and a cooldown phase (generally negative slope). In the heat-upphase, the heat-up rate may take on almost any value (e.g., in °F./minute). In the hold phase, the temperature and pressure may be heldsubstantially constant within prescribed bands or above prescribedminimums. In the cooldown phase, the cooldown rate (e.g., in °F./minute) may be limited, for example, by concerns regarding thermallyinduced stress, crystallinity issues, and/or equipment limitations. Inaddition or in the alternative, the pressure band may need to bemaintained until the temperature is significantly reduced.

FIG. 4A shows a temperature versus time profile for consolidating one ormore thermoplastic prepreg plies at a temperature sufficient to softenan aluminum layer, or for adjusting the temperature and pressure of astack so as to consolidate the stack when forming a stiffening element,according to some examples of the disclosed stiffening elements. Asshown in FIG. 4A, in the heat-up phase, the heat-up rate may take onalmost any value; in the hold phase, the consolidation temperature(e.g., T_(Consolidation)) may be held substantially constant within aprescribed band (e.g., 707° F.±9° F. or 710° F.±10° F.); and in thecooldown phase, the cooldown rate may be limited (e.g., ≤100° F./minute)until the temperature is significantly reduced (e.g., until thetemperature is ≤250° F.).

FIG. 4B shows a pressure versus time profile for consolidating one ormore thermoplastic prepreg plies at a temperature sufficient to softenan aluminum layer, or for adjusting the temperature and pressure of astack so as to consolidate the stack when forming a stiffening element,according to some examples of the disclosed stiffening elements. Asshown in FIG. 4B, in the heat-up phase, the pressure is low; in the holdphase, the consolidation pressure (e.g., P_(Consolidation)) may be heldsubstantially constant within a prescribed band (e.g., 100 psig±5 psigfor 20 minutes+15 minutes/−5 minutes or 290 psig minimum for 6 minutesminimum); and in the cooldown phase, the pressure band may need to bemaintained until the temperature is significantly reduced (e.g., 100psig±5 psig or 290 psig minimum until the temperature is ≤250° F.).

As shown in FIG. 4B, the pressure increase to the adjusted pressureand/or the pressure decrease from the adjusted pressure may berelatively rapid. The pressure increase to the adjusted pressure and/orthe pressure decrease from the adjusted pressure may be relatively slow,so that the leading and/or trailing edges of the pressure graph have amore gradual slope. The pressure increase to the adjusted pressureand/or the pressure decrease from the adjusted pressure may be conductedin a series of smaller steps.

FIG. 4C shows a temperature and pressure versus time profile forconsolidating one or more thermoplastic prepreg plies at a temperaturesufficient to soften an aluminum layer, or for adjusting the temperatureand pressure of a stack, according to some examples of the disclosedstiffening elements. As shown in FIG. 4C, in the heat-up phase, theheat-up rate may take on almost any value; in the hold phase, theconsolidation temperature (e.g., T_(Consolidation)) and theconsolidation pressure (e.g., P_(Consolidation)) may be heldsubstantially constant within prescribed bands or above prescribedminimums (e.g., 707° F.±9° F. and 290 psig minimum for 6 minutesminimum); and in the cooldown phase, the cooldown rate may be limited(e.g., ≤108° F./minute) until the temperature is significantly reduced(e.g., until the temperature is ≤410° F.) and/or the pressure minimummay need to be maintained until the temperature is significantly reduced(e.g., 290 psig minimum until the temperature is ≤248° F.).

During the heat-up phase, the heat-up may be paused to allowtemperatures to stabilize and/or standardize, and then the heat-up maycontinue (e.g., effectively creating a soaking or pre-consolidation stepor steps in the heat-up profile at a dwell temperature, not shown).Similarly, during the cooldown phase, the cooldown may be paused toallow temperatures to stabilize and/or standardize, and then thecooldown may continue (e.g., effectively creating a soaking orpost-consolidation step or steps in the cooldown profile, not shown).

Stiffening elements with a variety of component cross-sectional shapesmay be produced. The stiffening elements may have, for example,cross-sections that are round and solid (e.g., a rod), round and hollow(e.g., a tube), rectangular and solid, or rectangular and hollow. Thestiffening elements may have, for example, cross-sections that resembleblades, hats, the Greek capital letter Ω, and/or the English capitalletters C, I, L, T, U, or Z.

Example 1

A 4-layer stack may comprise: an aluminum layer (layer 1), two glasscomposite layers (sublayers 2-3 and 12-13), and a carbon composite layer(sublayers 4-11). The carbon composite layer may exhibit quasi-isotropicstrength properties. Per the table below, each glass composite layercomprises two sublayers, and the carbon composite layer comprises eightsublayers.

Layer Layer Layer Number Composition Thickness (inches) 1 Aluminum0.0100 2 Glass Composite 0.0035 3 Glass Composite 0.0035 4 CarbonComposite 0.0055 5 Carbon Composite 0.0055 6 Carbon Composite 0.0055 7Carbon Composite 0.0055 8 Carbon Composite 0.0055 9 Carbon Composite0.0055 10 Carbon Composite 0.0055 11 Carbon Composite 0.0055 12 GlassComposite 0.0035 13 Glass Composite 0.0035 Total N/A 0.0680

Example 2

A 4-layer stack may comprise: an aluminum layer (layer 1), two glasscomposite layers (sublayers 2-3 and 20-21), and a carbon composite layer(sublayers 4-19). The carbon composite layer may exhibit quasi-isotropicstrength properties. Per the table below, each glass composite layercomprises two sublayers, and the carbon composite layer comprisessixteen sublayers.

Layer Layer Layer Number Composition Thickness (inches) 1 Aluminum0.0100 2 Glass Composite 0.0025 3 Glass Composite 0.0025 4 CarbonComposite 0.0054 5 Carbon Composite 0.0054 6 Carbon Composite 0.0054 7Carbon Composite 0.0054 8 Carbon Composite 0.0054 9 Carbon Composite0.0054 10 Carbon Composite 0.0054 11 Carbon Composite 0.0054 12 CarbonComposite 0.0054 13 Carbon Composite 0.0054 14 Carbon Composite 0.005415 Carbon Composite 0.0054 16 Carbon Composite 0.0054 17 CarbonComposite 0.0054 18 Carbon Composite 0.0054 19 Carbon Composite 0.005420 Glass Composite 0.0025 21 Glass Composite 0.0025 Total N/A 0.1064

Example 3

A 4-layer stack may comprise: an aluminum layer (layer 1), two glasscomposite layers (sublayers 2-3 and 12-13), and a carbon composite layer(sublayers 4-11). The carbon composite layer may exhibit quasi-isotropicstrength properties. Per the table below, each glass composite layercomprises two sublayers, and the carbon composite layer comprises eightsublayers.

Layer Layer Layer Number Composition Thickness (inches) 1 Aluminum0.0100 2 Glass Composite 0.0035 3 Glass Composite 0.0035 4 CarbonComposite 0.0075 5 Carbon Composite 0.0075 6 Carbon Composite 0.0075 7Carbon Composite 0.0075 8 Carbon Composite 0.0075 9 Carbon Composite0.0075 10 Carbon Composite 0.0075 11 Carbon Composite 0.0075 12 GlassComposite 0.0035 13 Glass Composite 0.0035 Total N/A 0.0840

Example 4

A 4-layer stack may comprise: an aluminum layer (layer 1), two glasscomposite layers (layers 2 and 11), and a carbon composite layer(sublayers 3-10). The carbon composite layer may exhibit quasi-isotropicstrength properties. Per the table below, each glass composite layercomprises one layer, and the carbon composite layer comprises eightsublayers.

Layer Layer Layer Number Composition Thickness (inches) 1 Aluminum0.0100 2 Glass Composite 0.0035 3 Carbon Composite 0.0075 4 CarbonComposite 0.0075 5 Carbon Composite 0.0075 6 Carbon Composite 0.0075 7Carbon Composite 0.0075 8 Carbon Composite 0.0075 9 Carbon Composite0.0075 10 Carbon Composite 0.0075 11 Glass Composite 0.0035 Total N/A0.0770

E-glass, S-glass, and/or S-2 glass fibers (e.g., 933 S-2 glass fibers),for example, for glass composite layers may be commercially available,for example, from AGY of Aiken, S.C., under the trade name S-2 Glass®.

Carbon fibers (e.g., AS4D 4000 carbon fibers, IM7 (HS-CP-5000) carbonfibers), for example, for carbon composite layers may be commerciallyavailable, for example, from Hexcel Corporation of Stamford, Conn.,under the trade name HexTow®.

FIG. 5 shows stiffening element 500, according to some examples of thedisclosed stiffening elements. Stiffening element 500 may belightweight, strong, and comprise one or more integral current returnflowpaths. Thus, stiffening element 500 may be used as a lightweightstrength member, while also being incorporated into a CRN, among otheruses.

Stiffening element 500 may have a cross-section that resembles theEnglish capital letter “I”. Stiffening element 500 may have, forexample, another shape, such as a cross-section that resembles a blade,a hat, the Greek capital letter Ω, or the English capital letter C, L,T, U, or Z.

As shown in FIG. 5, the upper-left branch of stiffening element 500comprises (from top down): first aluminum layer 502; first glasscomposite layer 504 adjacent to first aluminum layer 502; first carboncomposite layer 506 adjacent to first glass composite layer 504, andopposite to first aluminum layer 502; second glass composite layer 508Aadjacent to first carbon composite layer 506, and opposite to firstglass composite layer 504; and second aluminum layer 510A adjacent tosecond glass composite layer 508A, and opposite to first carboncomposite layer 506. Any of these layers may have the same thickness asor a different thickness from any other of these layers.

As also shown in FIG. 5, the upper-right branch of stiffening element500 comprises (from top down): first aluminum layer 502; first glasscomposite layer 504 adjacent to first aluminum layer 502; first carboncomposite layer 506 adjacent to first glass composite layer 504, andopposite to first aluminum layer 502; second glass composite layer 508Badjacent to first carbon composite layer 506, and opposite to firstglass composite layer 504; and second aluminum layer 510B adjacent tosecond glass composite layer 508B, and opposite to first carboncomposite layer 506. Any of these layers may have the same thickness asor a different thickness from any other of these layers.

As also shown in FIG. 5, the lower-left branch of stiffening element 500comprises (from bottom up): third aluminum layer 512; third glasscomposite layer 514 adjacent to third aluminum layer 512; second carboncomposite layer 516 adjacent to third glass composite layer 514, andopposite to third aluminum layer 512; second glass composite layer 508Aadjacent to second carbon composite layer 516, and opposite to thirdglass composite layer 514; and second aluminum layer 510A adjacent tosecond glass composite layer 508A, and opposite to second carboncomposite layer 516. Any of these layers may have the same thickness asor a different thickness from any other of these layers.

As also shown in FIG. 5, the lower-right branch of stiffening element500 comprises (from bottom up): third aluminum layer 512; third glasscomposite layer 514 adjacent to third aluminum layer 512; second carboncomposite layer 516 adjacent to third glass composite layer 514, andopposite to third aluminum layer 512; second glass composite layer 508Badjacent to second carbon composite layer 516, and opposite to thirdglass composite layer 514; and second aluminum layer 510B adjacent tosecond glass composite layer 508B, and opposite to second carboncomposite layer 516. Any of these layers may have the same thickness asor a different thickness from any other of these layers.

As also shown in FIG. 5, the central branch of stiffening element 500comprises (from left to right): second aluminum layer 510A; second glasscomposite layer 508A adjacent to second aluminum layer 510A; firstcarbon composite sublayer 506-3A adjacent to second glass compositelayer 508A, and opposite to second aluminum layer 510A; third carboncomposite sublayer 518-1 (part of third carbon composite layer 518)adjacent to first carbon composite sublayer 506-3A, and opposite tosecond glass composite layer 508A; third carbon composite sublayer 518-2(part of third carbon composite layer 518) adjacent to third carboncomposite sublayer 518-1, and opposite to first carbon compositesublayer 506-3A; first carbon composite sublayer 506-3B adjacent tothird carbon composite sublayer 518-2, and opposite to third carboncomposite sublayer 518-1; second glass composite layer 508B adjacent tofirst carbon composite sublayer 506-3B, and opposite to third carboncomposite sublayer 518-2; and second aluminum layer 510B adjacent tosecond glass composite layer 508B, and opposite to first carboncomposite sublayer 506-3B. Any of these layers may have the samethickness as or a different thickness from any other of these layers.

First glass composite layer 504 may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. First carboncomposite layer 506 may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies. Second glass composite layer 508A/B maycomprise one or more glass-fiber-reinforced thermoplastic prepreg plies.Third glass composite layer 514 may comprise one or moreglass-fiber-reinforced thermoplastic prepreg plies. Second carboncomposite layer 516 may comprise one or more carbon-fiber-reinforcedthermoplastic prepreg plies. Third carbon composite layer 518 maycomprise one or more carbon-fiber-reinforced thermoplastic prepregplies.

First aluminum layer 502 may comprise a plurality of first aluminumsublayers 502-1, 502-2 (e.g., 502-1, 502-2, . . . , 502-n). The number(n) of first aluminum sublayers may be, for example, 2, 3, 4, 5, 6, ormore sublayers.

First glass composite layer 504 may comprise a plurality of first glasscomposite sublayers (e.g., 504-1, 504-2, . . . , 504-o). The number (o)of first glass composite sublayers may be, for example, 2, 3, 4, 5, 6,7, 8, 9, 10, 11, 12, or more sublayers.

First carbon composite layer 506 may comprise a plurality of firstcarbon composite sublayers 506-1, 506-2, 506-3A/B (e.g., 506-1, 506-2, .. . , 506-p). The number (p) of first carbon composite sublayers may be,for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or moresublayers (e.g., 8, 16, 24, 32, 40, or 64 sublayers).

Second glass composite layer 508A/B may comprise a plurality of secondglass composite sublayers (e.g., 508A/B-1, 508A/B-2, . . . , 508A/B-q).The number (q) of second glass composite sublayers may be, for example,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more sublayers.

Second aluminum layer 510A/B may comprise a plurality of second aluminumsublayers (e.g., 510A/B-1, 510A/B-2, . . . , 510A/B-r). The number (r)of second aluminum sublayers may be, for example, 2, 3, 4, 5, 6, or moresublayers.

Third aluminum layer 512 may comprise a plurality of third aluminumsublayers 512-1, 512-2 (e.g., 512-1, 512-2, . . . , 512-s). The number(s) of third aluminum sublayers may be, for example, 2, 3, 4, 5, 6, ormore sublayers.

Third glass composite layer 514 may comprise a plurality of third glasscomposite sublayers (e.g., 514-1, 514-2, . . . , 514-t). The number (t)of third glass composite sublayers may be, for example, 2, 3, 4, 5, 6,7, 8, 9, 10, 11, 12, or more sublayers.

Second carbon composite layer 516 may comprise a plurality of secondcarbon composite sublayers 516-1, 516-2 (e.g., 516-1, 516-2, . . . ,516-u). The number (u) of second carbon composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or more sublayers(e.g., 8, 16, 24, 32, 40, or 64 sublayers).

Third carbon composite layer 518 may comprise a plurality of thirdcarbon composite sublayers 518-1, 518-2 (e.g., 518-1, 518-2, . . . ,518-v). The number (v) of third carbon composite sublayers may be, forexample, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or more sublayers(e.g., 8, 16, 24, 32, 40, or 64 sublayers).

First aluminum layer 502 may be configured to form the outer surface ofstiffening element 500 or an outer surface of stiffening element 500.Second aluminum layer 510A/B (second aluminum layer 510A, secondaluminum layer 510B, or both) may be configured to form the outersurface of stiffening element 500 or an outer surface of stiffeningelement 500. Third aluminum layer 512 may be configured to form theouter surface of stiffening element 500 or an outer surface ofstiffening element 500.

Any of first aluminum layer 502, second aluminum layer 510A/B (secondaluminum layer 510A, second aluminum layer 510B, or both), or thirdaluminum layer 512 may be configured to form a first outer surface ofstiffening element 500, while any other of first aluminum layer 502,second aluminum layer 510A/B (second aluminum layer 510A, secondaluminum layer 510B, or both), or third aluminum layer 512 may beconfigured to form a second outer surface of stiffening element 500. Forexample, first aluminum layer 502 may be configured to form a firstouter surface of stiffening element 500, while third aluminum layer 512may be configured to form a second outer surface of stiffening element500. In another example, second aluminum layer 510A may be configured toform a first outer surface of stiffening element 500, while secondaluminum layer 510B may be configured to form a second outer surface ofstiffening element 500.

Stiffening element 500 may comprise one or more integral currentflowpaths. First aluminum layer 502 may be configured to form at leastpart of the one or more integral current flowpaths. Second aluminumlayer 510A/B (second aluminum layer 510A, second aluminum layer 510B, orboth) may be configured to form at least part of the one or moreintegral current flowpaths. Third aluminum layer 512 may be configuredto form at least part of the one or more integral current flowpaths.

Any of first aluminum layer 502, second aluminum layer 510A/B (secondaluminum layer 510A, second aluminum layer 510B, or both), or thirdaluminum layer 512 may be configured to form a first flowpath of the oneor more integral current flowpaths, while any other of first aluminumlayer 502, second aluminum layer 510A/B (second aluminum layer 510A,second aluminum layer 510B, or both), or third aluminum layer 512 may beconfigured to form a second flowpath of the one or more integral currentflowpaths. The first flowpath may differ from the second flowpath (e.g.,independent from each other). The first and second flowpaths may be partof a same integral current flowpath.

Various combinations of first aluminum layer 502, second aluminum layer510A/B (second aluminum layer 510A, second aluminum layer 510B, orboth), or third aluminum layer 512 may be configured to form one ormultiple (two, three, or four) flowpaths of the one or more integralcurrent flowpaths. The multiple flowpaths may differ from each other(e.g., independent from each other). Two or more of the multipleflowpaths may be part of a same integral current flowpath.

Current flow in any one of the multiple flowpaths may be substantiallyparallel to current flow in any other of the multiple flowpaths. Currentflow in any one of the multiple flowpaths may be substantially in a samedirection as current flow in any other of the multiple flowpaths.Current flow in any one of the multiple flowpaths may be substantiallyin an opposite direction from current flow in any other of the multipleflowpaths. Current flow in any one of the multiple flowpaths may besubstantially parallel to and in a same direction as current flow in anyother of the multiple flowpaths. Current flow in any one of the multipleflowpaths may be substantially parallel to but in an opposite directionfrom current flow in any other of the multiple flowpaths.

Each of first aluminum layer 502, second aluminum layer 510A/B (secondaluminum layer 510A, second aluminum layer 510B, or both), or thirdaluminum layer 512 may comprise the same aluminum alloy as any other offirst aluminum layer 502, second aluminum layer 510A/B (second aluminumlayer 510A, second aluminum layer 510B or both), or third aluminum layer512. Each of first aluminum layer 502, second aluminum layer 510A/B(second aluminum layer 510A, second aluminum layer 510B or both), orthird aluminum layer 512 may comprise a different aluminum alloy thanany other of first aluminum layer 502, second aluminum layer 510A/B(second aluminum layer 510A, second aluminum layer 510B or both), orthird aluminum layer 512.

Stiffening element 500 may comprise an additional layer (not shown)between the layers discussed above. When an additional layer is betweentwo other layers, the two other layers may be adjacent to each other,but not in direct contact. The additional layer may improve the bondingof the two other layers. The additional layer may at least partiallydecouple effects (e.g., thermal contraction, thermal expansion, strains,or stresses) associated with the bonding of the two other layers.

The additional layer may comprise, for example, an adhesive layer. Careshould be taken during selection of material(s) for such an additionallayer because, for example, some adhesives comprise silver or otherelements or compounds that may interact with, for example, aluminum,carbon, or glass via one or more interaction mechanisms (e.g., galvaniccorrosion).

The additional layer may comprise, for example, a so-called “radiusfiller” to fill in gaps between the two other layers associated with,for example, a bend or corner in stiffening element 500.

Methods of using stiffening element 500, for example, as part of acurrent return network for a stiffened composite structure, maycomprise: selecting stiffening element 500 that comprises one or moreintegral current flowpaths; and routing current from the current returnnetwork through the one or more integral current flowpaths of stiffeningelement 500.

In the methods of using stiffening element 500, the routing of thecurrent from the current return network comprises routing the currentfrom the current return network through first aluminum layer 502, secondaluminum layer 510A/B (second aluminum layer 510A, second aluminum layer510B, or both), and/or third aluminum layer 512.

In the methods of using stiffening element 500, stiffening element 500may comprise multiple (e.g., two, three, or four) integral currentflowpaths. The routing of the current from the current return networkmay comprise routing the current from the current return network throughthe two, three, or four integral current flowpaths. For example, in athree-integral-current-flowpath case, current may be routed in a firstdirection through second aluminum layer 510A/B, and in a seconddirection through both first aluminum layer 502 and third aluminum layer512. In another example, in a four-integral-current-flowpath case,current may be routed in a first direction through second aluminum layer510A, in a second direction through second aluminum layer 510B, in athird direction through first aluminum layer 502, and in a fourthdirection through third aluminum layer 512.

In the methods of using stiffening element 500, current flow in oneintegral current flowpath may be substantially parallel to and in a samedirection as current flow in any other integral current flowpath. Inaddition or in the alternative, current flow in one integral currentflowpath may be substantially parallel to but in an opposite directionfrom current flow in any other integral current flowpath.

Although examples have been shown and described in this specificationand figures, it would be appreciated that changes may be made to theillustrated and/or described examples without departing from theirprinciples and spirit, the scope of which is defined by the followingclaims and their equivalents.

What is claimed is:
 1. A stiffening element that comprises one or moreintegral current flowpaths, the stiffening element comprising: a firstlayer comprising a plurality of carbon-fiber-reinforced thermoplasticplies; a second layer, adjacent to the first layer, comprising one ormore glass-fiber-reinforced thermoplastic plies; and a third layer,adjacent to the second layer and opposite to the first layer, comprisingaluminum; wherein the third layer is configured to form an outer surfaceof the stiffening element, and wherein the third layer is configured toform at least part of the one or more integral current flowpaths.
 2. Thestiffening element of claim 1, wherein the third layer comprises 1000series aluminum alloy.
 3. The stiffening element of claim 1, wherein thethird layer comprises 1100 aluminum alloy.
 4. The stiffening element ofclaim 1, wherein the plurality of carbon-fiber-reinforced thermoplasticplies comprises thermoplastic resin.
 5. The stiffening element of claim4, wherein the thermoplastic resin comprises polyetheretherketone (PEEK)or polyetherketoneketone (PEKK).
 6. The stiffening element of claim 1,wherein the one or more glass-fiber-reinforced thermoplastic pliescomprise thermoplastic resin.
 7. The stiffening element of claim 6,wherein the thermoplastic resin comprises polyetheretherketone (PEEK) orpolyetherketoneketone (PEKK).
 8. The stiffening element of claim 1,wherein the plurality of carbon-fiber-reinforced thermoplastic pliescomprises first thermoplastic resin, wherein the one or moreglass-fiber-reinforced thermoplastic plies comprise second thermoplasticresin, and wherein the first thermoplastic resin is the same as thesecond thermoplastic resin.
 9. A stiffening element that comprises oneor more integral current flowpaths, the stiffening element comprising: afirst layer comprising a first aluminum layer; a second layer, adjacentto the first layer, comprising one or more first glass-fiber-reinforcedthermoplastic plies; a third layer, adjacent to the second layer andopposite to the first layer, comprising a plurality ofcarbon-fiber-reinforced thermoplastic plies; a fourth layer, adjacent tothe third layer and opposite to the second layer, comprising one or moresecond glass-fiber-reinforced thermoplastic plies; and a fifth layer,adjacent to the fourth layer and opposite to the third layer, comprisinga second aluminum layer; wherein the fifth layer is configured to form afirst outer surface of the stiffening element, and wherein the fifthlayer is configured to form at least part of the one or more integralcurrent flowpaths.
 10. The stiffening element of claim 9, wherein thefirst layer is configured to form a second outer surface of thestiffening element.
 11. The stiffening element of claim 9, wherein thefirst and second aluminum layers comprise a same aluminum alloy.
 12. Thestiffening element of claim 9, wherein the first and second aluminumlayers comprise different aluminum alloys.
 13. The stiffening element ofclaim 9, wherein the first layer is configured to form at least part ofthe one or more integral current flowpaths.
 14. The stiffening elementof claim 9, wherein the first layer is configured to form at least partof a first flowpath of the one or more integral current flowpaths,wherein the fifth layer is configured to form at least part of a secondflowpath of the one or more integral current flowpaths, and wherein thefirst flowpath differs from the second flowpath.
 15. The stiffeningelement of claim 14, wherein current flow in the first flowpath issubstantially parallel to and in a same direction as current flow in thesecond flowpath.
 16. The stiffening element of claim 14, wherein currentflow in the first flowpath is substantially parallel to but in anopposite direction from current flow in the second flowpath.
 17. Amethod of using a stiffening element that comprises one or more integralcurrent flowpaths as part of a current return network for a stiffenedstructure, the method comprising: selecting the stiffening elementcomprising a first layer that comprises a first aluminum layer, a secondlayer adjacent to the first layer that comprises one or more firstglass-fiber-reinforced thermoplastic plies, a third layer adjacent tothe second layer and opposite to the first layer that comprises aplurality of carbon-fiber-reinforced thermoplastic plies, a fourth layeradjacent to the third layer and opposite to the second layer thatcomprises one or more second glass-fiber-reinforced thermoplastic plies,a fifth layer adjacent to the fourth layer and opposite to the thirdlayer that comprises a second aluminum layer, wherein the fifth layer isconfigured to form a first outer surface of the stiffening element, andwherein the fifth layer is configured to form at least part of the oneor more integral current flowpaths; and routing current from the currentreturn network through the one or more integral current flowpaths of theselected stiffening element.
 18. The method of claim 17, wherein therouting of the current from the current return network comprises routingthe current from the current return network through the fifth layer. 19.The method of claim 17, wherein the stiffening element comprises firstand second integral current flowpaths, and wherein the routing of thecurrent from the current return network comprises routing the currentfrom the current return network through the first and second integralcurrent flowpaths.
 20. The method of claim 19, wherein current flow inthe first integral current flowpath is substantially parallel to and ina same direction as current flow in the second integral currentflowpath, or wherein the current flow in the first integral currentflowpath is substantially parallel to but in an opposite direction fromthe current flow in the second integral current flowpath.